The second stage of development of the bones of the base of the skull. Skeleton development stages
In the development of the skull, both cerebral and facial, three stages are distinguished: membranous, cartilaginous and bone. These stages are temporary for humans and higher mammals. Passing from one to another, they correspond to permanent forms in phylogeny. The membranous stage of development of the skull in humans begins from the end of the 2nd week of the embryonic period, the cartilaginous stage from the 2nd month. The end of both the membranous and cartilaginous stages and, consequently, the beginning of the bony stage in different parts of the skull is different. So, for example, the ossification point in the lower jaw appears on the 39th day, and in the main part of the occipital bone - on the 65th day of intrauterine development. These three stages pass through the bones involved in the formation of the base of the skull (except for the medial plates of the pterygoid process of the sphenoid bone), and not most of bones of the facial section of the skull. Most of the bones of the facial skull and roof pass the cartilaginous stage. The bony stage in these parts of the skull follows the membranous stage. In some bones (occipital, temporal), some parts develop as primary bones, others as secondary. On this basis, the bones of the skull are divided by origin into primary - integumentary, developing on the basis of connective tissue, and secondary - lining, arising in the place of cartilage. Primary bones include: the upper part of the scales of the occipital, parietal, frontal, scales of the temporal bone, tympanic ring, inner plate of the pterygoid process of the sphenoid, palatine, vomer, nasal, lacrimal, zygomatic bones, upper and lower jaw.
The secondary bones of the human skull include: the occipital (except for the upper part of the scales), wedge-shaped (without the inner plate of the pterygoid process), ethmoid and concha, pyramid and mastoid temporal, auditory ossicles (malleus, incus, stirrups) and the body of the hyoid bone.
The skull develops partly from the preexisting cranial part of the dorsal cord and its derivative, partly from the derivatives of the branchial arches. The bony skull is formed after the formation of the brain, nerves and blood vessels and forms around them. This is the reason for the formation of a large number of holes and channels in the skull, which serve for the passage of blood vessels and nerves.
Development of the cerebral section of the skull... The formation of the skull in mammalian embryos begins with an accumulation of mesenchyme around the dorsal cord (chord) at the level of the hindbrain, from where it spreads under the anterior and upper parts of the brain, forming the basis for developing brain and its roof. This primary mesenchymal cover of the skull later turns into a connective membranous tissue, desmocranium (membranous stage of development of the skull). Areas of the membranous skull persist in separate locations after birth as fontanelles. At 2-4 months of development, a number of cartilages appear around the anterior end of the dorsal string: parachordal, cartilagines parachordales (occipital, cartilago occipitalis, wedge-shaped, cartilagines sphenoidalis, etc.)? as well as cartilaginous cases, containers of the organs of smell, sight and hearing: nasal, visual and auditory capsules (Fig. 20). Parachordal cartilage penetrates to the site of the future pituitary gland. The above individual cartilages merge with each other as they develop, as well as with the nasal, visual and auditory capsules. The result is a solid cartilaginous plate of the base of the skull, chondrocranium, with a median opening for the pituitary gland. At this stage (the second half of the 3rd month), the skull is a formation in which there is a cartilaginous base in the form of a narrow groove. The rest of the skull is connective tissue (cartilaginous or primary skull).
At the next stage of development of the skull, ossification of the cartilaginous base and membranous roof occurs and the formation of the bony skull, osteocranium. The process of development and formation of the bones of the skull, like the rest of the bones of the skeleton, occurs in a certain sequence. According to certain periods of intrauterine development of the fetus, ossification nuclei appear in the connective tissue and cartilaginous anlages of future bones. Spreading in depth and over the surface of the ossification nuclei merge with each other, forming the outer and inner plates of the compact bone substance and the spongy substance located between them.
Not all cartilage formations are ossified. A number of cartilages remain in adults (the cartilage of the alar nasal, the cartilaginous part of the nasal septum and the small cartilages of the base of the skull).
Different parts of some bones develop differently: some - in place of cartilage, others - in place of connective tissue (for example, the occipital bone develops as a primary bone, the rest - as secondary bones). With age, parts of the bones and individual bones fuse together, due to which the total number of bones in the skull decreases.
Facial development of the skull... The facial part of the skull develops mainly from the branchial arches. In aquatic vertebrates, the gill arches are located metamerically in the intervals between the gill slits through which water passes to the gills - the respiratory organs of these animals.
In vertebrates living on land, gill slits are present only in the embryonic period. The number of branchial arches varies in different aquatic animals: in terrestrial vertebrates, they are laid in an amount of 6; a person develops 5 arches, and the 5th arch is poorly developed (rudimentary). 1st (upper) is called the mandibular, or mandibular, arch. The 2nd - the sublingual, or hyoid, arch, and the rest - respectively, the 3-5th branchial arches.
The development of the facial skull involves the 1-3rd branchial arches and the frontal process, which limits the mouth bay from above - the future oral cavity (Fig. 21). The 1st branchial arch on each side forms two processes - the maxillary and mandibular, which limit the mouth bay from below and from the sides. The maxillary processes are separated from each other by the frontal process, which in the process of development is divided into three parts: unpaired (middle) and paired (lateral). The organ of vision is placed between the maxillary process and the lateral part of the frontal. The lacrimal groove is located between the lateral parts of the frontal and maxillary processes. The mandibular processes of the right and left arches grow together.
The formation of the upper and lower jaws occurs around the cartilaginous primordia of the mandibular arch, of which one is called dorsal, cartilago dorsalis, and the second, ventral, cartilago ventralis. In lower fish, these two sections of the mandibular arch serve as jaws. In vertebrates, these cartilages are models around which the ossifying mesenchyme grows, forming the upper and lower jaws. The maxillary process gives rise to the upper jaw (with the exception of the part corresponding to the region of the incisors of the intermaxillary bone), the zygomatic bone, the palatine, the medial plate of the pterygoid process, the sphenoid bone. The mandibular process gives rise to the mandible, which develops periosteally around the vanishing Meckelian cartilage. The middle part of the frontal process forms the vomer, the perpendicular plate of the ethmoid bone and the intermaxillary bone. The lateral part of the frontal process serves to form the labyrinth of the ethmoid bone, nasal and lacrimal bones. In addition to these bones, the 1st branchial arch gives rise to the hammer and the incus, the 2nd branchial arch forms the stapes, the styloid process, the small horns of the hyoid bone, the 3rd branchial arch is the body and large horns of the hyoid bone.
The skull is the most complex part of the skeleton. Its design is determined by the development of the head end of the neural tube and anterior section intestinal tube. There are 2 sections in the skull:
The cerebral division (neurocranium), which is the receptacle of the brain and sense organs. It is subdivided into a convex roof or cranial vault, calvaria, and a flattened base, basis cranii.
Facial section (splanchnocranium), which forms the bone base of the initial sections of the digestive and respiratory systems. It contains the eye sockets, the nasal cavity and the oral cavity.
The cerebral and facial parts of the skull in phylogeny and ontogeny are formed relatively independently, although anatomically closely related to each other.
The adult skull consists of 23 permanent bones. The cerebral section is formed by - unpaired bones - frontal, occipital, wedge-shaped; paired bones - parietal and temporal. The facial section is formed by - paired bones: nasal, lacrimal, upper jaw, zygomatic, palatine bone, lower turbinate; unpaired bones: vomer, lower jaw, hyoid bone. The ethmoid bone is part of both the brain and the facial regions.
A feature of the skull is the presence of bones in it, in which there are cavities containing air. These bones are called pneumatic. These include the frontal, wedge-shaped, ethmoid, temporal bones, and the upper jaw.
The skull is viewed in several positions called norms.
Facial norm- a front view of the skull, allows you to see the anterior part of the fornix - the forehead, the eye sockets, the pear-shaped opening leading to the nasal cavity, the upper and lower jaws and their alveolar parts, in which the teeth are located.
Lateral (lateral) norm- a side view of the skull, gives the most vivid idea of \u200b\u200bthe ratio of the brain and facial regions, as well as the vault and base of the skull. In the lateral norm, you can see all the bones of the brain and most of the bones of the facial regions.
Vertical norm- a view of the skull from above, gives an idea of \u200b\u200bthe shape of the cranial vault and its constituent bones - frontal, parietal and occipital. In this position, the coronary, sagittal and lambdoid sutures, frontal and parietal tubercles are visible.
Occipital norm- a rear view of the skull, showing the occipital and parietal bones. In the occipital norm, one can see the lambdoid and mastoid-occipital sutures, the external occipital eminence, the nuchal lines, the mastoid processes.
Basilar norm- a view of the skull from below, demonstrates the outer base of the skull, with bony formations located on it, as well as the bony palate.
Skull evolution
The cerebral skull was formed in vertebrates as a continuation of the axial skeleton of the body. In lower vertebrates, the skull is built of cartilage, which forms the cerebral box, ear and nasal capsules. The cerebral box is subdivided in relation to the notochord into chordal and prechordal divisions. The border between them corresponds to the position of the pituitary gland. This is the primary, or primordial, skull. It is most developed in cartilaginous fish (sharks, rays). A feature of the primordial skull is the continuity of the cartilage from which it is built.
The next stage of craniogenesis is the formation of the bony skull. In the cartilaginous tissue, foci of ossification appeared and separate bones were formed, separated by layers of cartilage. Bone tissue was found in the skull of the most ancient cross-finned and lungfish. Already at this stage of evolution, the skull contained two types of bones. Some were formed by replacing cartilage with bone tissue (replacement bones), others were formed in the membranous tissue that covers the top of the brain (integumentary bones).
The latter, according to the theory of A.N. Severtsov, originated from the skin ossifications of ancient fish. As a result of the fusion of numerous spiked scaly plates, the bones of the skull roof were formed, which were very numerous at first. Later, they merged with each other and advanced on the primordial skull, partially covering it from the outside.
The cerebral section of the skull in phylogeny expanded in the caudal direction. This can be judged by the exit cranial nerves... If in cyclostomes the last nerves emerging from the skull are the VII and VIII pairs of nerves (facial and vestibular cochlear), then in amphibians there are already 10 pairs of cranial nerves, and in mammals the number of cranial nerves reaches 12. Expanding, the skull probably assimilated the rudiments of the cervical vertebrae, but this can only be judged indirectly.
In lower vertebrates, the visceral skull constitutes the skeleton of the branchial arches and devices for grasping food. In terrestrial vertebrates, the visceral skull undergoes significant transformations due to the fact that, with the emergence of animals on land, the gill respiration was replaced by the pulmonary one, and the gill apparatus lost its former significance. As a result, there was a restructuring of its elements into organs with a different function. In particular, many bones of the facial region were formed from the gill arches.
The evolutionary transformations of the cerebral section took place in the direction of increasing its capacity and increasing strength. The latter was achieved by progressive ossification and fusion of individual bone elements among themselves, as a result of which the total number of bones in the skull decreased significantly. This is clearly seen when comparing the skulls of bony fish and extinct amphibians - stegocephals with the skulls of mammals. Mammals are characterized by almost complete ossification of the skull with the formation of large bony complexes at its base - the occipital, temporal and sphenoid bones. In mammalian evolution, there has been a significant increase in the capacity of the cerebral box. So, in fish, the ratio of the facial skull to the brain is 6: 1, in the horse - 4.5: 1; for a monkey - 1: 1; in humans - 1: 2. In modern apes, the capacity of the cerebral skull reaches 500 cm 3.
The skull has undergone great changes during the stages of anthropogenesis. They are due to the progressive development of the brain, upright posture, weakening of the load on the dentoalveolar apparatus, and the development of speech.
The capacity of the cerebral skull of the ancient great apes - Australopithecus, who lived in southern and eastern Africa, ranged from 413 to 516 cm 3 in their various forms. In the East African man of skill, homo habilis, the age of the remains of which is estimated at 1.5-2 million years, it was equal to an average of 645 cm 3. Among the Pithecanthropus, who lived on the island of Java 600-500 thousand years ago, the capacity of the skull was about 900 cm 3, and for the synanthropus belonging to a later period, it reached 1000 cm 3. The capacity of the cranial cavity in Homo sapiens, homo sapiens, exceeds 1100 cm 3.
Along with the increase in the cerebral section in anthropogenesis, its rounding took place and the ratio with the facial skull changed. The cerebral skull moved towards the facial one, so that the longitudinal axis of the latter moved in relation to the base of the skull. If in animals the facial axis forms an angle slightly less than 180º with the base of the skull, then in humans they are located almost at right angles. Formed a bend of the base of the skull in its middle part, the so-called "basilar angle". In connection with the rearrangement of the posterior part of the skull due to the straightening of the body, the foramen magnum and the occipital condyles moved to the base of the skull.
Significant changes have also taken place in the facial area. They are expressed primarily in the reduction of the jaws and alveolar processes. As a result of this, a chin protrusion was formed, which represents one of the specific features of the human face. Another characteristic human symptom was the reduction of the nasal section and the formation of the external nose. There was an increase in the eye sockets and their turn forward.
As a result of the described changes, the human skull is almost balanced at the atlanto-occipital joint. Thanks to this, the muscles attached to the skull were largely freed from the function of maintaining the head in a balanced state and fine movements became possible at the joints of the skull with the spinal column.
In the process of anthropogenesis, the gracilization of the skull took place, that is, a decrease in its massiveness: There was a weakening of the bone relief, the superciliary arches and occipital protuberances decreased, the frontal scales acquired a more vertical position, the bones of the skull became thinner, and the skull itself was less massive.
8. The brush as an organ of labor. (see Question No. 18).
10-11. Development of the cerebral and facial skull. Skull and intracranial pressure in ontogeny. Derivatives of the visceral arches.
There are two parts to the skull: cerebral (cerebral) and facial (extracerebral). The brain part consists from the vault and base, which are formed by several bones, while the bones of the vault develop in place of connective tissue and go through two stages - membranous and bone (primary bones), bypassing the cartilaginous, and the bones of the base, developing in place of connective tissue, go through three stages: webbed , cartilaginous (secondary bones) and bone. Facial skull bones are formed in connection with the development of the branchial arches (first and second), which are the basis of the facial part of the head, and in its development, part of the bones goes through three stages, the other part - two (connective tissue and bone). Occipital bone (with the exception of the upper part of the scales) - secondary bone, has four enhodral centers of ossification, all of them are concentrated around the foramen magnum: two on the sides, one in front, one behind. The upper part of the scales - the primary bone, has two points of ossification, on both sides of the median plane. Full accretion of all parts occurs in the 4th-6th year of life. Parietal bone - primary, its bony points appear in the area of \u200b\u200bfuture parietal tubercles at the end of the 10th week of the prenatal period, while the direction of bone tissue growth is radial in relation to the parietal tubercle. The upper and lower temporal lines begin to form by the age of 12-15. Frontal bone - primary, develops from two points of ossification, each of which appears in the area of \u200b\u200bthe future supraorbital margins at the end of the 9th week of the intrauterine period. At birth frontal bone consists of two halves, the fusion of which along the middle plane, starting at the 6th month after birth, ends by the end of the 3rd year in the form of a metopic suture, which disappears by the age of 8. Frontal sinuses begin to appear in the first year of life. Sphenoid bone - secondary (with the exception of the medial plate of the pterygoid process and the lateral-superior portions of the large wings), develops from the enchondral nuclei, which appear symmetrically at the following times: in the area of \u200b\u200bsmall wings, in the area of \u200b\u200blarge wings, in the body of the bone under the pituitary fossa - at the 3rd month ; in the area of \u200b\u200bthe carotid sulcus and uvula - at the beginning of the 4th month; in the front of the body - at the end of the 4th month; from two pairs of endesmal nuclei: in the area of \u200b\u200bthe medial plates of the pterygoid processes at the 3rd month and in the area of \u200b\u200bthe lateral-upper sections of the large wings at the end of the 3rd month of the prenatal period. Complete ossification of the sphenoid bone occurs in the 10th year of life. The development of the sinuses begins in the 3rd year of life. Temporal bone develops from the following points of ossification: endesmal centers of the scales appear at the beginning of the 3rd month, and the tympanic cavity - at the end of the 3rd month of the prenatal period; enchondral ossification points appear for the pyramid at the 5th month of the prenatal period and for the styloid process - at the end of the first year of life. The tympanic part as such in the newborn is absent and represents a ring, which begins to ossify in the middle of the 3rd month of the prenatal period. As for the cells of the mastoid process, they are finally formed by 5-6 years. Complete ossification of the temporal bone ends by 6 years. Ethmoid bone - secondary, develops from cartilage and ossifies with several points. The earliest points of ossification appear in the middle turbinate - at the 4th month of intrauterine development and in the superior turbinate - at the 5th month. Then at the 9th month, two nuclei for the ethmoid plate appear. At the 6th month after birth, the ossification nucleus of the orbital plate is formed, which ossifies very quickly. In the second year of life, two nuclei of ossification appear, one on each side, of the future cock's comb, which, later merging, form a cock's comb. At the 6-8th year of life, the perpendicular plate ossifies, and by the age of 12-14, the lattice cells of the labyrinth are finally established. Inferior turbinate - secondary, has one core of ossification, which appears at the beginning of the third month of the prenatal period. Lacrimal bone - primary, develops from one point of ossification, appearing on the 3rd month of the prenatal period. Coulter - primary bone, develops from two endosteal centers of ossification, arising during the second month of the prenatal period, each of them located parallel to the median plane. In the future, the right and left plates grow together, and the cartilage of the wasp septum located between them is absorbed after birth. Upper jaw - primary bone, develops from 5 zdesmal centers of ossification: external upper and external lower, internal anterior and internal posterior and middle. The outer superior nucleus forms the medial part of the orbital fundus. The outer lower core gives rise to the outer part of the bottom of the orbit, the zygomatic process, the antero-outer part of the body and the posterior-outer wall of the alveolar process. The middle nucleus develops into the frontal process and the part of the body below it. From the inner posterior nucleus, the posterior 2 /, palatine process and the inner wall of the alveolar process are formed, respectively, to the canine and molars. From the inner anterior point of ossification, the incisor bone is formed - the part of the alveolar process corresponding to the incisors, and the anterior third of the palatine process. At the 5th month, the nuclei merge, and the newborn retains the incisor suture that connects the incisor bone with the rest of the upper jaw. The sinuses of the upper jaw, appearing at the 6th month of the prenatal period, complete their development by the age of 12-14. Palatine bone - primary, develops from one point of ossification, which appears in the second month of the prenatal period at the junction of the perpendicular and horizontal plates. Cheekbone - primary, formed from one point of ossification, which appears at the end of the second month of the prenatal period. The lower jaw develops as a steam room, and is mixed in its development - its processes, condylar and coronary, passing through the stage of cartilage, are secondary, the rest goes through the stage of membranous ossification, it is primary. Each half of the lower jaw, in the form of a groove, encloses the cartilage of the first branchial arch, which dissolves by the 5th month of the prenatal period, while its distal portion forms the chin bone, and the proximal end of the cartilage serves as the basis for the development of the auditory ossicles. The bony connection of both halves begins at the 3rd month after birth and ends at the age of two. Hyoid bone - secondary, develops from 5 points: one for the body and one in each large and small horns. Ossification points in the body and great horns appear at the end of the prenatal period or shortly after birth; small horns ossify by 13-15 years. The fusion of large horns with the body occurs rather late, by the age of 30-40, sometimes even later, the fusion of small horns occurs towards old age. The age differences of the skull as a whole, its topographic areas and individual bones are expressed primarily in different ratios of the sizes of the brain and facial regions. These differences, as well as the thickness of the bones, the size of the pits and cavities of the skull, the presence of fontanelles and synostosis of the sutures of the skull, etc., are determined by the growth and development of the skull over five periods. The first period, from birth to 7 years, is characterized by active growth of the skull, especially its volume, and the seams are somewhat narrowed and the size of the fontanelles gradually decreases, nasal cavity and the eye sockets, increasing, are formed; the relief of the lower jaw changes noticeably. In the second period, which lasts up to 14 years, the change in the size and shape of the skull and its parts is not as active as in the first period, however, the pits, mastoid, orbital and nasal cavities noticeably increase. The third period covers the age from puberty to 25 years. At this time, the frontal regions are formed and the facial skull is lengthened, the area of \u200b\u200bthe zygomatic arches is noticeably increased, the tubercles protrude more. The fourth period, up to 45 years, is characterized primarily by the fact that ossification of the seams, which began at 20-30 years, ends by the end of this period. It is noted that premature fusion of the sagittal suture leads to the formation of short, and the coronal suture - long skulls. The fifth period runs from 45 years to old age and is characterized by atrophy of the facial and then cerebral skull, a gradual decrease in the number of teeth, which affects the shape of the jaws. Further, the alveolar processes are smoothed out, the entire skull becomes smaller.
Skull and intracranial pressure in ontogenesis. The first base of the head is the dorsal string. The brain is surrounded by a lining of mesenchyme - the membranous skull. For 2 months. cartilage tissue appears, in the region of the anterior end of the notochord. On both sides, a longitudinal cartilaginous strip is formed, and anteriorly, a pair of cartilaginous plates, Rathke's cartilaginous beams. The plates merge, forming the cartilaginous base of the skull with the chord enclosed in it. The brain is covered from above only by the connective tissue sheath. In the middle of the 2nd month. uterine life begins the development of the bones of the skull.
Group I - secondary bones of the cerebral skull: occipital, wedge-shaped, pyramids and mastoid part of the temporal bone.
II gr. - CMB integumentary: frontal, parietal, upper part of the occipital scales, scales, tympanic part of the temporal bone.
III gr. - the bones of the nasal capsule are secondary: ethmoid, inferior concha.
IV gr. - KNK integumentary: lacrimal, nasal, opener.
V gr. - the bones of the jaw area are integumentary: the upper jaw, palatine bone, zygomatic bone and medial plate of the processus pterygoideus.
VI gr. - bones of the visceral arches: the lower jaw, malleus, incus, stirrup, styloid process of the styloid bone and hyoid bone.
Visceral arches.
I visceral arch - jaw (mandibular). Derivatives - malleus, incus, Meckel's cartilage, maxillary-hyoid muscle, anterior abdomen of the digastric muscle.
II visceral arch - sublingual (hyoid). Derivatives - stirrup, styloid process of the temporal bone, small horns of the hyoid bone, stylohyoid ligament, stylohyoid muscle, posterior belly of the digastric muscle, subcutaneous muscle.
The regulation of intracranial pressure is carried out: 6 fontanelles, cerebrospinal fluid (presence of intershell spaces), arterial and venous systems - the nature of their interaction (outflow due to glomus caroticum).
| Parameter name | Value |
| Topic of the article: | DEVELOPMENT OF THE SKULL |
| Category (thematic category) | Education |
The skull goes through 3 stages of development: membranous, cartilaginous and bony. The membranous and cartilaginous stages are temporary for higher mammals and humans. Οʜᴎ pass one into the other and to some extent correspond to permanent forms in phylogeny. The membranous stage in humans begins from the end of the 2nd week of the embryonic period, the cartilaginous - from the 2nd month. The timing of the onset of the bone stage and, therefore, the end as a membrane
that and cartilaginous stage in different parts of the skull is different. So, in the lower jaw, the ossification point appears on the 39th day, and in the basilar part of the occipital bone - on the 65th day of intrauterine development. In the cerebral skull, bones or parts of bones involved in the formation of the base of the skull go through 3 stages of development. In the bones of the cranial vault, the bony stage immediately follows the membranous stage. Most of the bones of the facial skull also pass the cartilaginous stage, and only a few of them go through all 3 stages of development. By origin, all bones of the skull are divided into primary, developing from connective tissue, and secondary, arising from the cartilaginous bone model.
Primary bones: the upper part of the occipital scales, scaly and tympanic parts of the temporal bone, parietal and frontal bones, medial plate of the pterygoid process of the sphenoid bone, palatine bone, vomer, nasal, lacrimal, zygomatic bones, upper and lower jaws. Secondary bones: occipital (except for the upper part of the occipital scales), sphenoid (without the medial pterygoid plate), ethmoid bones, inferior turbinates, pyramid and mastoid of the temporal bone, auditory ossicles (malleus, incus, stirrup) and the body of the hyoid bone.
The skull develops on the basis of the cranial part of the notochord and mesenchyme surrounding it and the rudiment of the brain, as well as from derivatives of the branchial ducts. The membranous skull has a series of holes and channels for the passage of nerves and blood vessels, and the future occipital bone has a large opening for the spinal cord. With the further development of the brain, nerves and blood vessels, the bony skull is formed around them, due to which many openings and channels are formed in it, which serve for the passage of blood vessels and nerves (Table 1).
The development and formation of the bones of the skull, like the rest of the bones of the skeleton, occur in a certain sequence. Centers (points) of ossification appear in the membranous and cartilaginous anlages of future bones at the appropriate time. Spreading over the surface and in depth, they merge with each other and form the outer and inner plates of the compact bone substance and the spongy substance located between them. Not all cartilaginous formations of the skull are ossified. In adults, the cartilage of the wings of the nose, the cartilaginous parts of the septum of the nose and the small cartilages of the base of the skull are preserved.
Table 1. The timing of the appearance of centers of ossification in the bones of the skull (according to BM Patten, 1959)
The bones of the skull in the fetus and newborn are larger than in an adult. The number of bones decreases as a result of the fusion of several bones into one. The newly formed bone can consist of parts of different origins, ᴛ.ᴇ. primary bones are connected to secondary ones. For example, the scales of the occipital bone develop as the primary bone, and the rest as secondary bones.
Development of the cerebral skull
Skull formation begins with an accumulation of mesenchyme around the notochord at the level of the hindbrain. From here, the mesenchyme extends beneath the anterior and upper parts of the brain, forming the base and vault of the developing brain. This primary mesenchymal cover subsequently develops into a membranous skull (leptocranium). After birth, areas of the membranous skull remain as fontanelles. The cartilaginous stage begins at 2-4 months of intrauterine development, when parachordal and prechordal cartilaginous beams appear around the anterior end of the notochord, as well as cartilaginous sheaths - the receptacles of the organs of smell, vision, hearing (nasal, visual and auditory capsules) (Fig. 7) ... Parachordal
Figure: 7. Development of the skull (2-3rd month of embryogenesis):
a - top view: 1 - nasal capsule; 2 - visual capsule; 3 - prechordal cartilage; 4 - auditory capsule; 5 - parachordal cartilage; 6 - chord; b - left view: 1 - pituitary fossa; 2 - parachordal cartilage; 3 - chord; 4 - III cervical vertebra; 5 - body of the II cervical vertebra; 6 - anterior arch of the I cervical vertebra
Figure: 8. Development of the cerebral skull; cartilaginous skull (second half of the 3rd month): 1 - cartilaginous skull
cartilage penetrates to the location of the future pituitary gland. As development progresses, individual cartilages merge with each other, as well as with the nasal, visual and auditory capsules, due to which a continuous cartilaginous plate with a median opening for the pituitary gland is formed on the base of the skull (Fig. 8). In this period (the second half of the 3rd month), the skull has a cartilaginous base and a membranous vault - the so-called cartilaginous skull (chondrocranium) is formed. At the next stage of the development of the skull, ossification of the membranous vault and cartilaginous base and the formation of the bony skull, osteocranium, occur (Fig. 9-11).
Figure: 9. Ossification of the cranial vault:
1 - brain bubbles; 2 - the point of ossification of the scales of the occipital bone; 3 - cartilaginous skull; 4 - point of ossification of the frontal scales
Figure: 10. The next stage of ossification of the cranial vault:
1 - primary bones of the cranial vault;
2 - cartilaginous skull
Figure: 11. Centers of ossification of the occipital (a, b), sphenoid (c) and temporal (d) bones
Development of the facial skull
The bones of the facial skull develop from the branchial arches and the frontal process, limiting the mouth from above - the future oral cavity. In aquatic vertebrates, the gill arches are located metamerically in the intervals between the gill slits through which water passes, washing the gills - the respiratory organs. In vertebrates living on land, in the embryonic period, branchial arches (clusters of mesenchyme) are formed, and instead of gill slits, branchial pockets are formed between them. In terrestrial vertebrates, there are 6 branchial arches, in humans there are 5 of them, and the 5th branchial arch is poorly developed (rudimentary). The 1st branchial arch is usually called the mandibular arch, and the rest are called the 2nd to 5th branchial arch, respectively. The 1-3rd are involved in the development of the facial skull
branchial arches (Fig. 12). The branchial pocket between the 1st and 2nd branchial arches in humans differentiates into the middle ear cavity and the auditory tube.
Figure: 12. The initial stage of facial development; embryo 5-6 weeks:
a - side view: 1 - frontal tubercle; 2 - the rudiment of the eye; 3 - mandibular (1st) branchial arch; 4 - auditory vesicle; 5 - 2nd (hyoid) branchial arch; 6 - 3rd and 4th branchial arches; 7 - the first branchial pocket; 8 - heart protrusion; 9 - mouth bay;
b - front view: 1 - frontal tubercle; 2 - the medial nasal process; 3 - lateral nasal process; 4 - maxillary process of the 1st branchial arch; 5 - the mandibular process of the 1st branchial arch; 6 - mouth bay; 7 - 3rd and 4th branchial arches; 8 - 2nd branchial arch; 9 - 1st branchial pocket; 10 - nasolacrimal groove; 11 - eye rudiment; 12 - olfactory fossa.
The mandibular branchial arch on each side forms 2 processes - the maxillary and mandibular, which limit the mouth bay from below and from the sides. Between the maxillary processes there is a frontal process, which is divided into 5 parts during the formation of the olfactory fossa: an unpaired frontal process and paired lateral and medial nasal processes. The medial nasal process forms the vomer, the perpendicular plate of the ethmoid bone and the intermaxillary bone (usually exists as a separate bone before birth). The lateral nasal process is the source of the formation of the labyrinth of the ethmoid bone, nasal and lacrimal bones. The maxillary and lateral nasal processes limit the orbital cavity,
which continues downward and medially into the lacrimal-nasal groove, which connects to the olfactory fossa. Subsequently, the nasolacrimal groove closes, forming on each side the nasolacrimal canal (Fig. 13). From the maxillary process, the upper jaw (with the exception of the area corresponding to the incisors), palatine and zygomatic bone, the medial plate of the pterygoid process of the sphenoid bone. The palatine processes extend from the medial surfaces of the maxillary processes. Οʜᴎ are connected with each other and with the septum of the nasal cavity, forming the palate (Fig. 14).
Figure: 13. Further stages of face formation in embryogenesis:
a - 7 weeks of embryonic development, front view; b - the same, right side view;
c - 8 weeks of embryonic development, front view; d - the same, right side view
Figure: 14. Development of the palate, bottom view:
a - 6-7th week: 1 - frontal tubercle;
2 - the medial nasal process; 3 - lateral nasal process; 4 - lacrimal-nasal groove; 5 - the maxillary process of the mandibular branchial arch; 6 - the palatine process of the maxillary process; 7 - the base of the skull - the roof of the mouth bay; 8 - growing septum of the nasal cavity; b - 7-8th week: 1 - filtrum - the place of fusion of the medial nasal processes; 2 - the place of fusion of the medial nasal and maxillary processes;
3 - primary palate; 4 - primary choans; 5 - palatine processes of the maxillary processes; 6 - growing septum of the nasal cavity;
c - 8-10th week: 1 - upper lip; 2 - gum; 3 - secondary palate
The lower jaw develops from the mandibular process periosteally (Fig. 15). The 1st branchial arch also gives rise to the hammer and the incus, the 2nd branchial arch - the stirrups, the styloid process of the temporal bone, the small horns of the hyoid bone, the 3rd branchial arch - to the body and large horns of the hyoid bone (see Fig. 15).
Figure: 15.Development of the lower jaw, auditory ossicles and laryngeal cartilage:
1 - hammer; 2 - anvil; 3 - stirrup; 4 - subulate process of the temporal bone; 5 - styloidmandibular ligament; 6 - large horns of the hyoid bone; 7 - thyroid cartilage of the larynx; 8 - arytenoid cartilage of the larynx; 9 - cricoid cartilage of the larynx; 10 - small horns of the hyoid bone;
11 - chin bone; 12 - lower jaw
The individual ossifications, from which the bones of the skull develop, continue to grow together with each other only after birth.
Especially significant are the remains of the connective tissue membranous skull in the walls of the cerebral skull where several bones converge. Here, in a newborn, soft areas of the skull are felt, which are called fontanelles... There are six of them: two unpaired and two paired on each side.
The biggest - front - fontanelle, which has a diamond shape; otherwise it is called the big fontanelle. It is located where the right and left halves of the frontal bone and the right and left parietal bones converge. Another unpaired - tasks- the fontanelle is placed where the right and virgin parietal bones and the occipital bone converge.
On the left and right sides, there are lateral paired fontanelles. The front one is wedge-shaped - located where the frontal, parietal bones converge and big wing sphenoid bone. Posterior lateral (or mastoid) the fontanel is located in the place where the occipital, parietal bones and the mastoid process of the temporal bone converge.
After birth, as a result of the growth of the edges of the cranial bones, cranial sutures are formed: in the area of \u200b\u200bthe facial skull - even with smooth edges, in the area of \u200b\u200bthe cerebral skull - dentate, between the temporal and parietal bones - a scaly suture. When the sutures are formed between the smooth, serrated and scaly edges of the cranial bones, fibrous connective tissue containing bone-forming elements is preserved. The main role of the cranial sutures is that bone growth occurs along the edges.
Simultaneously with the formation of sutures, ĸᴏᴛᴏᴩᴏᴇ ends in the 3-5th year of life, fontanelles close. In this case, the corners of the parietal bone are formed. Earlier than others, at the 2nd month of life, the mastoid fontanelle closes. Later, all - only by 3 years - the wedge-shaped fontanelle is finally closed. The anterior fontanelle, which closes by 1.5 years of age, is of greatest practical importance. It is this fontanelle that is for pediatricians an indicator of the course of ossification of the skull and skeleton as a whole. Failure to close the anterior fontanelle after 1.5 - 2 years of life indicates a delay in the ossification of the skull, and, consequently, the entire skeleton.
Not only in structure, but also in shape, the skull of a newborn is different from that of an adult. The proportions, the ratio of the cerebral and facial skull in an adult and a newborn are different. The newborn has a relatively large cerebral and relatively small facial skull. In an adult, the facial skull is relatively larger. In terms of mass and volume, the brain through the newborn is 8 times larger than the facial, in an adult - only 2 times.
Τᴀᴋᴎᴍ ᴏϬᴩᴀᴈᴏᴍ, in intrauterine life, the cerebral skull grows more intensively than the facial one; the opposite is observed after birth. This is due to the fact that the growth and formation of the cerebral skull are determined primarily and mainly by the growth and development of the brain. In higher mammals and humans, the development of the brain already in the early stages is characterized by the progressive growth of the anterior region. The active growth of the brain also determines the progressive growth of the cerebral skull in comparison with the facial skull.
In a newborn, the upper part of the facial skull is most developed, since the eyeball grows very vigorously during the prenatal period. The eyeball is a derivative of the brain. Bottom part of the facial skull in uterine life lags behind in growth, since its formation is associated with the respiratory and chewing functions, which, naturally, have a formative influence only after birth. These functions take effect only after birth. Naturally, the facial skull grows and forms mainly after birth.
If we return from the individual development of the human skull to the evolution of the skull in vertebrates, it should be noted that in the course of evolution, the brain's skull recedes into the background. In amphibians, the facial skull is located directly anterior to the cerebral skull. The same position relative to each other of the facial and cerebral skulls is occupied by most mammals. Only in higher mammals, the facial skull, as it were, recedes back and moves under the cerebral skull. This can be seen on the skull of a baboon and even better on the skull of apes (chimpanzees, gorillas).
In modern humans, the brain skull is 2 times larger than the facial one.
During the prenatal period, the cerebral skull grows more strongly than the facial one, and, in addition, is relatively large in size than the skull of an adult. After birth, the cerebral skull lags behind the facial one. The facial skull, which in a newborn, so to speak, is hidden under the cerebral skull, protrudes anteriorly after birth - prognathism increases. The facial angle decreases after birth. This shows that in the ontogeny of the skull, which generally repeats evolution, the course of development shifts in time based on those formative influences that the function has on the developing organism and its individual parts. Since the respiratory and chewing functions are not realized in uterine life, the facial skull lags behind in growth in this period. When, after birth, these functions come into play, the facial skull overtakes the brain in development. This leads to a change in the proportion of the skull in the postnatal period. Thus, ontogeny does not blindly repeat phylogeny. Development is determined by the conditions of existence of the organism, the functions of the organs, and the entire organism of the embryo or the growing animal as a whole.
After birth, growth occurs unevenly not only in the area of \u200b\u200bthe facial and cerebral skull, but also in various parts of the cerebral and facial skull, as well as in time.
Three periods of skull growth can be distinguished: 1) a period of vigorous active growth - from birth to 7 years, 2) a period of slow growth - from 7 years to puberty, i.e. 14-16 years, 3) a new period of active growth of the skull - from the age of puberty to 25-26 years, when the growth of the entire skeleton ends.
The first period of vigorous growth of the skull, in turn, is divided into three segments. During the 1st year of life, the skull grows more or less evenly in all directions. From 1 to 3 years of age, the skull grows especially actively behind. The occipital part of the skull is forming; in the 2nd - 3rd year of life, the nape becomes convex. At the same time, that part of the skull that is located posterior to the foramen magnum increases, and the opening itself changes its position. If in a newborn the large occipital foramen is located in the transverse plane of the body, then in the 2nd - 3rd year of life it seems to be open downward and forward and is located obliquely, as if open downward and forward, its front edge becomes higher than the rear one.
Strong growth of the occipital skull and a change in the position of the foramen magnum are associated with the transition of the child to upright posture in the 2nd year of life. To a certain extent, this repeats the course of the evolution of the skull. In four-legged mammals, the head is suspended from the spinal column in front and is supported by powerful occipital muscles and ligaments: the foramen magnum is directed posteriorly. In primates, due to the fact that they lead an arboreal lifestyle and at times take an upright position, the skull, in order to maintain balance, begins to take a new position. At the same time, in monkeys, with an increase in the relative size of the cerebral skull, the large occipital foramen turns downward and gradually settles, as can be seen in antromorphic monkeys (chimpanzees, gorillas), obliquely, open downward and backward. The continuation of this process of formation of the occipital region of the skull and the displacement of the large occipital foramen to the lower surface of the skull occurred in human ancestors, in particular in Pithecanthropus. In an erect person, the skull is balanced in balance at the upper end of the spinal column and the foramen magnum is located so that it is open not only downward, but also anteriorly. Changes in the phylogeny of the location of the foramen magnum are, as it were, repeated in the individual development of a person.
In the 2nd - 3rd year of life, due to the eruption of milk teeth, the growth of the facial skull in height and width increases significantly, which also affects the development of the initial section respiratory tract, i.e. nasal cavity. At this age, the parietal bones are actively growing. Vertex, ĸᴏᴛᴏᴩᴏᴇ flat in newborn, becomes convex; from the end of the 3rd year, the seams will form.
During the third segment of the first growth period of the skull - from 3 to 7 years - the growth of the entire skull continues, with a particularly active growth of its base. By the age of 7, the growth of the base of the skull in length basically ends. The base of the skull reaches almost the same size as that of an adult.
In the second period of development of the skull - from 7 years to puberty (14 - 16 years) - the growth of the skull slows down; mainly the vault of the cerebral skull grows.
In the third period - from puberty and up to 25-26 years - the frontal part of the cerebral skull expands and deepens, the facial skull is also actively growing. In this burden, the sexual characteristics of the skull are manifested: in young men, the facial skull grows in length more strongly than in girls; the face is lengthened. If the faces of boys and girls are rounded before puberty, then after the onset of puberty in women, the face retains its characteristic roundness, and in men, as a rule, it stretches.
There is a great deal of literature on the difference between the male and female skulls. A number of features were put forward that distinguish the male and female skulls. Today it is generally accepted that the features of the male and female skulls consist, firstly, in the sex differences in the size of the skull: the male skull, due to the large overall body size, is larger than the female one. The skull capacity in men is 1559 cubic meters. cm, women have 1347 cubic meters. cm, ᴛ.ᴇ. on average, men have 212 cubic meters. see more than women. Moreover, if we calculate the relative capacity of the skull per 1 cm of body length, it turns out that in women it is greater; than men.
If we compare the development of the cerebral and facial skulls, it turns out that the cerebral skull is relatively strongly developed in women, and the facial one is more significant in men. The female skull, as it were, largely retains the features of the children's skull of our ancestors. For women, a smaller protrusion anteriorly of the facial skull is characteristic, the female skull is less prognathic than the male. This is associated with the features of the profile line of the male and female skulls. In men, the facial skull protrudes relatively more strongly; the profile line of the forehead, rising gently upward, passes into the convex crown and continues with the convex line of the occiput. In women, the facial skull protrudes slightly anteriorly, and therefore the profile line of the forehead is directed vertically upward. It, bending sharply, passes into the flat line of the crown, and the latter, with a sharp bend, into the downward profile line of the flattened occiput. As a rule, the male skull is distinguished by a more pronounced relief of muscle attachments; in women, the relief of the skull is less pronounced.
So, the growth of the skull after birth continues unevenly in its individual parts and in certain periods of postnatal life. Starting from the period of puberty, the sexual characteristics of the formation of the skull are determined. What are the mechanisms of skull growth?
The bones of the skull, like the entire skeleton, grow through apposition, that is, the imposition of new layers bone plates on an existing bone. The cranial bones grow through the apposition of new bone substance to the surface of the bones, as well as in the cranial sutures and synhodroses. Naturally, after the growth of the cranial bones ends, the function of the cranial sutures stops and they close, that is, they are overgrown with bone tissue. It starts at 25 - 40, most often at 30.
The seams of the skull are overgrown with bone tissue in a specific sequence. First, the cranial sutures are overgrown from the inside of the cranial cavity, that is, the internal compact plates of the bones grow together, and later the sutures are closed outside. At the age of 22 - 35 years, the sagittal suture is closed, at 24 - 38 years old - the coronary suture in the middle part, at 26 - 41 years old - in its lower part, at 26 - 42 years old the lambdoid suture. Later than others, the masto-occipital (from 30 years old) and scaly (from 37 years old) sutures are closed.
The time and order of overgrowth of the cranial sutures are individually variable. With increased intracranial pressure for one reason or another, the overgrowth of sutures is delayed until very old age. So, it is known that the cranial sutures of the famous philosopher of the second half of the 18th century Kant did not grow until 80 years old.
If we compare, for example, the lower jaw of a newborn and an adult, it becomes clear that if the lower jaw grew only by uniformly superimposing bone substance on its entire surface, then the jaw of an adult could never have been formed from the jaw of a newborn. In the process of growth, the cranial bones, like other bones of the skeleton, not only increase, but also change shape. On the other hand, comparing the skulls of a newborn and an adult, it can be seen that the skull of a newborn fits freely in the craniocerebral cavity of an adult. If the growth of the skull consisted only in the fact that new bone substance was superimposed on its bones, then the cranial cavity could not increase. Measuring the shape of the bones and enlarging the cranial cavity are possible only because during the growth of the skull, on the one hand, a new bone substance is applied by apposition, on the other hand, the previously formed bone tissue is destroyed. In the process of skull growth, conflicting processes of neoplasm and destruction of bone tissue are intertwined.
Microscopic studies show that, for example, the parietal bone grows so that on its outer surface and along its edges in the seams, osteoblasts form all new rows of bone plates, and on the inner surface of the bone, osteoclasts destroy bone tissue... Microscopic examination of the growing mandible revealed that simultaneously with the imposition of new bone substance along the posterior edge of the jaw ramus at the anterior edge of the ramus of the lower jaw, the bone substance is destroyed. Using the example of the lower jaw, one can trace how the shape of the bone changes during the growth of the skull. In the fetus, the body and the branch of the lower jaw are on one straight line, that is, they form an angle of 180 °. In a newborn, they are also located almost on the same straight line and form an angle of 150 °. The structure of the lower jaw is such that most of the body of the bone is occupied by bone cells in which the dental sacs are placed, but there are no alveoli or holes yet. Only when the teeth erupt does the alveolar process form, the alveolar edge forms. Taking into account the dependence of the action of the masticatory muscles on the lower jaw, the body of the lower jaw grows and becomes taller. In connection with teething and a change in the relationship between the upper and lower jaws, the mechanical conditions of movement in the mandibular joint, the direction of the masticatory muscle bundles and the traction of the masticatory muscles change. In this regard, the angle between the body and the branch of the lower jaw decreases and in an adult it ranges from 120 to 130 °.
In old age, the shape of the lower jaw changes again. After the teeth fall out, the alveolar process is absorbed and only a narrow arch of the lower jaw body remains. The chewing function gradually fades away, the muscles weaken. With the weakening of the traction of the muscles, their shaping influence on the bone is lost.
Taking into account the dependence on the loss of teeth, violation of the relationship between the upper and lower jaws, changes in the chewing muscles, the shape of the lower jaw changes again. The angle between the body and the arch of the lower jaw increases again and reaches 140 °. The lower jaw, as it were, again takes on the shape characteristic of a newborn. But this impression is wrong, the similarity of the shape of the lower jaw of a child and an old man is purely external. In a newborn, the entire lower jaw is filled with dental sacs, and in old people the alveolar process completely disappears and only a narrow arch of the jaw body remains.
Senile changes are found not only in the lower jaw, but also in the entire skull. Due to the fact that the muscles weaken, the relief of the skull is smoothed, the bumps and roughness are absorbed. The skull becomes lighter. There is a partial resorption of the spongy substance, especially the flat cranial bones. The crown is somewhat flattened, since the outer and inner plates of the compact substance of the parietal bones come closer together. At the same time, the elasticity of the skull decreases, it becomes fragile. In an adult, the skull is elastic, when squeezed in the transverse direction, its width can decrease by 1 cm. Old anatomists performed the following experiment in front of an audience of students: a young man's skull was thrown onto a stone floor, and he, possessing elasticity, bounced like a ball; then the old man's skull was thrown, and he crumbled to pieces.
So, throughout the entire life of a person (in the uterine period and after birth until very old age), the shape and structure, strength and elasticity of the skull are continuously changing. The continuous change in the shape and structure of the skull is determined by the continuous change in its functions as a support and protection for the soft parts of the head and face.
The primary role in the development of the skull and its shaping belongs to the brain. The progressive development of the brain in the course of the evolution of vertebrates causes a change in the ossification of the cerebral skull. In human ontogenesis, the early progressive development of the cerebral skull depends on the development of the brain in the early stages of uterine life. The influence of the brain on the formation of the skull determines many features of the relief of the inner surface of the cranial box. With scoliosis, ᴛ.ᴇ. lateral curvature of the spinal column, the right and left halves of the skull base bear unequal stress. On the side of the curvature of the spinal column, the imprints of the cerebral convolutions are deeper. This natural experiment supports the opinion about the decisive role of the influence of the formation of the brain on the growth and development of the skull.
Other bodies and soft tissue heads ( eyeball, chewing muscles, glands, contents of the nasal cavity, etc.) also affect the shape of the skull. Numerous experiments, in particular carried out at the end of the last century by the famous Russian anatomist PF Lesgaft and his students, show that the growth and formation of the skull are determined by the influence of the soft parts of the head and face. The experimenters removed the eyeball and the entire contents of the orbit from young puppies. The puppies grew up, they were slaughtered and the shape of the skull of the operated puppies was compared with the shape of the skull of the control - normally growing puppies of the same litter. After removal of the contents of the orbit, the latter on the operated side decreased in size, and the shape of the bones involved in its construction changed. Moreover, the shape of the entire cerebral skull also changed: the corresponding half of the cranial cavity increased, proceeding from the predominant growth of the cerebral hemispheres in the direction of least resistance, that is, on the side where the contents of the orbit were removed. The skull became asymmetrical. Likewise, the shape of the facial skull and the entire configuration of the head in puppies change after removal of the turbinates. After such an operation, the shape of the skull of the puppies of the Pointing Dog changed so much that it began to resemble the skull characteristic of dogs of another breed - pugs.
The chewing muscles have a great influence on the formation of the skull. Removal of the temporal muscle causes the development of asymmetry of the cerebral and facial skull.
The role of mechanical influences on the formation of the skull was studied by P.F. Lesgaft and his students. Οʜᴎ bandaged puppies' skulls in different directions and got different skull shapes, incl. navicular and tower.
The influence of the mechanical factor on the development of the skull is also proved by observations of people. In the middle of the last century, travelers noticed that some Indian tribes in the territory of modern Mexico believe that a man, in order to be a good hunter and a winner in battles, must have a tall, tower-shaped skull. In order to make their son a good hunter and warrior, the parents took measures to develop the tower shape of the skull. Already in infancy, the boy's head was placed not on a pillow, but on a log, ĸᴏᴛᴏᴩᴏᴇ was placed under the back of the child's head. The skull only grew upward. Holding the child in her lap, the mother put her hand on his forehead and held it so that it pressed on the skull.
In the vicinity of Toulouse (France) in the past centuries, a head stretched to the back of the head was considered beautiful for a girl. Parents bandaged the girls' heads in the transverse direction, which achieved the desired shape of the skull - the so-called Toulouse head. The back of the head was pulled up and back: this was emphasized by a special hairstyle. This head shape in the indicated area was considered beautiful.
The variety of internal and external factors, which influence the growth and formation of the skull, make it clear that the shape and size of the skull have very large individual differences. Individual differences in the shape of the skull are studied by a special branch of anthropology, which is called craniology. This science uses the method of direct observation of the skull - cranioscopy and the method of measuring the skull - craniometry. The skull is examined from above - in the vertical norm, from below - in the basilar norm, from the side - in the lateral norm, from behind - in the occipital norm and in the sagittal cut, i.e., in the median norm. Individual differences in the shape of the cerebral and facial skull are determined along the contour of the skull in different rates. For example, in the vertical norm, a rhomboid, spheroid, pentagonoid, etc. skull is distinguished, comparing the outlines of the skull with various figures. Craniometrically determine the average and extreme variants of the absolute value of certain sizes of the skull. For example, it is known that, on average, among Russians inhabiting the middle zone of the European part of Russia, the greatest length of the skull is 175 mm, width 142 mm, height 133 mm.
At the same time, the shape of the skull is characterized not only by the absolute dimensions, in different directions, but also, most importantly, by the ratio of these dimensions. These ratios are expressed in multiple indexes or indices. The most common is the head index. It represents the greatest width of the cranial skull, expressed as a percentage of length. This index characterizes the ratio of the length and diameter of the skull. If the length of the skull is taken as 100, and the width is expressed as a percentage of the length, you get a relative number characterizing the shape of the skull. The index from 75 to 80 indicates the average value of the index - medium-headed, mesocephalic skull, less than 75 - long-headed, dolichocephalic, and more than 80 - brachycephalic, short-headed. The head index is one of the signs of physique, which together make it possible to distinguish between human races within humanity as a composite species.
For decades, fascist anthropologists have sought to prove the superiority of the North Germanic - dolichocephalic race over the Slavic - brachycephalic (average head index of the Slavic race is 81). Οʜᴎ believed that the highest race is the North German, or Nordic. It was proved that only this race, characterized by a dolichocephalic skull, possesses the signs of a higher structure of the skull. Some English and American scholars preached the superiority of the Anglo-Saxon race over others. At the same time, anthropological studies show that this or that skull shape does not belong to any one race. A greater degree of long-headedness than that of the North Germanic race is found in Negroes. On the other hand, the head indicator varies over a significant period within the same population. In all races of modern mankind, gradual brachycephaly occurs, the head index changes towards brachycephaly (round-headedness). The shape of the head also changes based on the conditions of existence of the same person for a relatively short time. American anthropologists, using the example of migrants from Italy and Poland to America, have shown that the head index changes under the influence of the external environment.
If the shape of the nose of Negroes is more similar to the shape of the nose of human ancestors, then the shape of the lips is more differentiated than that of Europeans. The white race retained a more developed hairline of the skin. Racial characteristics, incl. head index have no adaptive meaning.
Τᴀᴋᴎᴍ ᴏϬᴩᴀᴈᴏᴍ, it is wrong, comparing the structure of the human skull with the structure of the skull of great apes and other primates, to draw a conclusion about the higher or lower position of individual races on the evolutionary ladder. Humanity is represented by one species - Homo sapiens.
Finally, it is completely unscientific to associate certain features of the shape of the head or different sizes of the capacity of the skull, or even different mass of the brain with the degree of development of higher nervous activity.
About how great the individual differences in the shape and structure of the skull, how significant the variability of the structure and shape of the skull, how untenable are attempts to connect intellectual development with the shape of the skull, evidenced by the fact that even each individual person has the right
SKULL DEVELOPMENT - concept and types. Classification and features of the category "SKULL DEVELOPMENT" 2017, 2018.
And it is formed by the bones of the skull and bones of the face. The bones of the skull, in turn, form the cranial vault and base, which develop in different ways.
The bones of the vault are membranous in development, that is, they are formed directly in the embryonic skeletal mesenchyme. The membranous bones of the skull include the parietal bones, frontal scales, scaly and tympanic parts of the temporal bone, the wings of the sphenoid bone, the upper part of the occipital scales.
Most of the bones of the base of the skull develop on the basis of the anterior cartilage, i.e. are cartilaginous.
Face bonesother than palatine, and auditory bones, ossicula auditiva, are formed from the material of the branchial arches.
fig. 115 Skull of a newborn; view from above.Each of the bones of the skull and face has certain developmental characteristics and therefore is described separately.
Occipital bone, os occipitale, arises from points of ossification, concentrating around the large (occipital) foramen. At the 6th week of embryonic development, two points of ossification appear in front of the foramen, at 8-9 weeks - two on the sides and three points behind the occipital foramen, and the development proceeds according to the type of endochondral ossification. Until the moment of fusion of all four parts of the bone, they are separated by cartilage. The cartilage between the basilar and the lateral is named intraoccipital synchondrosis, synchondrosis intra-occipitalis, which distinguishes anterior intraoccipital synchondrosis, synchondrosis intra-occipitalis anterior (paired), and between the lateral parts and the occipital scales - posterior intraoccipital synchondrosis, synchondrosis intra-occipitalis posterior... At the junction of the basilar part and the body of the sphenoid bone is located sphenoid-occipital synchondrosis, synchondrosis spheno-occipitalis. The complete connection of bone parts begins at 2-4 years old and ends at 8-10 years old. The fusion of the basilar part of the occipital bone with the sphenoid body ends by the age of 20. The upper part of the occipital scale develops from two points of ossification, appearing on both sides of the median plane.
Parietal bone, os parietale, develops from two points of ossification, which appear in the area of \u200b\u200bfuture parietal tubercles at the 8-10th week of intrauterine development and merge with each other. In this case, the process of ossification proceeds radially with respect to the parietal tubercle. After birth, the angles of the parietal bones are absent, and the edges of the bone are separated by wide layers of connective tissue. Ossification ends only in the 2nd year of life. The upper and lower temporal lines clearly begin to form by the age of 12-15.
Frontal bone, os frontale, develops as a membranous, with the exception of the nasal region, which forms on the basis of cartilage. At the 8-9th week of intrauterine development, paired ossification points appear in the area of \u200b\u200bfuture tubercles and supraorbital margins, connecting into one bone at the age of 7-8 years. In this regard, at birth, the frontal bone consists of two halves, the fusion of which along the midplane begins from the 6th month after birth and ends in the 3rd year with the formation metopic suture, sutura metopicalasting up to 5 years.
Sphenoid bone, os sphenoidale, develops almost entirely on the basis of cartilage. The bone is formed from ossification points that appear at the end of the 2nd month of embryo development in the cartilaginous anlage of the bone body (anterior and posterior points), in each of the wings and in the medial plate of the pterygoid processes. Small wings connect to the body of the bone at 6-7 months, and large wings after birth.
fig. 210. Skull bones, ossa cranii (newborn). 1 - occipital bone os occipitale, outside view; 2 - occipital bone, os occipitale, inside view (a - sphenoid-occipital synchondrosis, synchondrosis spheno-occipitalis; b - anterior intraoccipital synchondrosis, synchondrosis intraoccipitalis anterior; c - posterior intraoccipital synchondrosis, synchondrosis intraoccipitalis posterior synchondrosis petro-occipitalis); 3 - sphenoid bone, os sphenoidale; 4 - temporal bone, os temporale; 5 - upper jaw, maxilla; 6 - lower jaw, mandibulla. Ethmoid bone, os ethmoidale, develops as cartilaginous. Earlier, ossification points appear in the middle (at the 4th month of intrauterine development) and in the upper (at the 5th month) turbinates. Then, at the 9th month, two points of ossification of the ethmoid plate appear. At the 6th month after birth, the ossification point of the orbital plate is formed. The latter ossifies very quickly. In the 2nd year of life, two points of ossification appear above the lattice plate, which later, merging, form a cock's comb. At the 6-8th year of life, the perpendicular plate ossifies, and by the age of 12-14, the lattice cells of the labyrinth are finally formed.
The sinuses of the bones of the skull are formed during the development of bone cells and cavities into which the mucous membrane grows. So, during the formation of the frontal sinus, the mucous membrane grows from the side of the ethmoid cells, and when the sphenoid sinus is formed, from the side of the nasal cavity.
Temporal bone, os temporale, is formed from four anlages, giving rise to scaly, tympanic and stony parts. Ossification points appear in the scaly part at the beginning, and in the tympanic part at the end of the 3rd month, in the stony part - at the 5th month of the prenatal period, and in the styloid process - at the end of the 1st year of life. The ear canal in a newborn is not yet formed, since the tympanic part forms an incomplete ring (see Fig. 97). In the first years of life, this ring grows and, together with the scaly part, forms the bony part of the external auditory canal. Complete ossification of the temporal part ends by 6 years.
Lower nasal concha, concha nasalis inferior, - cartilaginous bone. It develops from one point of ossification, which appears at the beginning of the 3rd month of the prenatal period.
Lacrimal bone, os lacrimale, membranous, develops from one point of ossification, appearing in the 3rd month of the prenatal period.
Coulter, vomer, - webbed bone. It develops from two - right and left - ossification points that arise during the 2nd month of the prenatal period. Subsequently, the right and left plates grow together, and the cartilage of the nasal septum located between them is absorbed after birth.
Upper jaw, maxilla, - webbed bone. It develops from 5 points of ossification: external (upper and lower), internal (anterior and posterior) and middle. The outer upper point forms the medial part of the bottom of the orbit, the outer lower gives rise to its outer part, the zygomatic process, the anterior-outer part of the bone body and the posterior-outer wall of the alveolar process. The midpoint develops into the frontal process and part of the body. From the inner posterior point, the posterior 2/3 of the palatine process and the inner wall of the alveolar process are formed, respectively, to the canine and molars. From the inner anterior point of ossification, the incisor bone is formed - the part of the alveolar process corresponding to the incisors, and the anterior part of the palatine process. At the 5th month, the ossification points merge, and the newborn retains the incisor suture that connects the incisor bone with the rest of the upper jaw. The sinuses of the upper jaw, appearing at the 6th month of the prenatal period, are finally formed by the age of 12-14.
Palatine bone, os palatinum, webbed. It develops from one point of ossification, which appears in the 2nd month inside the uterine period at the junction of the perpendicular and horizontal plates.
Zygomatic bone, os zygomaticum, also webbed. It is formed from one point of ossification, which appears at the end of the 2nd month of the prenatal period.
Lower jaw, mandibula, mixed in development: its processes, condylar and coronary, are cartilaginous, the rest develops as membranous. The bone is laid as a steam room. Each half of its wine in the form of a groove surrounds the cartilage of the first branchial arch, which dissolves by the 5th month of the prenatal period, while the lower part of the groove forms the chin bone, and the upper end of the cartilage serves as the basis for the development of the auditory ossicles. Both halves begin to connect at the 3rd month after birth, with the formation of the mental symphysis. Complete fusion of the bone parts ends by the age of two.
Hyoid bone, os hyoideum, secondary, develops from 5 points: from one the body is formed, and from the others - large and small horns. Ossification points in the body of the great horns appear late in the uterus or shortly after birth; small horns ossify by 13-15 years. The fusion of large horns with the body occurs rather late, by the age of 30-40, sometimes later, and the small horns grow together with the body of the hyoid bone by old age.
The age differences of the skull as a whole, its topographic areas and individual bones are expressed primarily in different ratios of the sizes of the brain and facial regions. These differences, as well as the thickness of the bones, the size of the pits and cavities of the skull, the presence of fontanelles and synostosis of the sutures of the skull, etc., are determined by the growth and development of the skull. There are 5 periods of skull development. The first period - from birth to 7 years - is characterized by active growth of the skull, an intensive increase in its volume. In this case, the seams are somewhat narrowed and the value fontanelles, fonticuli... The cavities of the nose and eye sockets are formed; the relief of the lower jaw changes noticeably. In the second period - from 7 to 14 years - the change in the size and shape of the skull and its parts is not as active as in the first, however, the pits, mastoid process, orbital and nasal cavities noticeably increase. The third period covers the age from puberty to 25 years. At this time, the frontal parts are formed and the facial skull is lengthened, the area of \u200b\u200bthe zygomatic arches is noticeably increased, the frontal tubercles protrude more. During the fourth period - from 25 to 45 years - ossification of the seams occurs. Observations have shown that premature ossification of the sagittal suture leads to the formation of short, and the coronal suture - long skulls. The fifth period - 45 years and more - is characterized by atrophy of the facial, and then the cerebral skull, a gradual decrease in the number of teeth, which affects the shape of the jaws: the alveolar processes and parts are smoothed, the angle of the lower jaw increases, the facial skull decreases in size.