Stages of skull development. Development and age-related characteristics of the skull bones Stages of development of anatomy as a science
Historical stages in the development of anatomy.
Stages of development of anatomy as a science
To understand the state and prospects for the development of any science, including anatomy, it is necessary to know the main stages of its formation. The history of anatomy, which is part of the history of medicine, is the history of the struggle between materialistic ideas about the structure of the human body and idealistic and dogmatic ones. The desire to obtain new, more accurate information about the structure of the human body, to know “oneself” for many centuries met resistance from the reactionary secular authorities and the church.
History of the development of anatomy in 5 periods:
1) Primary: covers the 5th century BC. Scientists: Heraclitus. Hippocrates (20 volumes).
2) Scientific development: covers from the 4th century BC. and until the 15th century AD. Scientists of the 4th century: Aristotle (Organ system. Theory of organism development). In the 2nd century BC: Galen. (States that the heart, brain, liver are the main organs).
3) Renaissance period: 16th century - and the first half of the 17th century. Scientists: Harvey - the doctrine of blood circulation. Fallopium - studies the structure of the genital organs. Eustachius - respiratory organs and hearing aid. Leonardo Da Vinci - images of animals and humans.
4) Microscopic period: second half of the 17th and 18th centuries. Scientists: Malpighius - the study of blood vessels.
5) Comparative embryonic period: XIX-XX centuries. - today's time. Scientists: Cuvier's theory of embryonic development.
Contribution of N.I. Pirogov in the development of human anatomy.
N.I. Pirogov (1810 – 1881) – creator of topographic anatomy. He introduced into anatomy the method of cutting frozen corpses to study the exact relative positions of organs. An outstanding surgeon, he attached great importance to knowledge of anatomy. Nikolai Ivanovich Pirogov was one of the founders of military field surgery, and also made an invaluable contribution to the development of topographic anatomy. Thanks to the activities of N.I. Pirogov, medicine in general and anatomy in particular, made a giant leap in their development. N.I. Pirogov (1810-1881) achieved enormous success in the development of surgical anatomy. His work “Surgical Anatomy of Arterial Trunks and Fascia” (1837) created his worldwide fame. He introduced a new research method into anatomy - successive cuts of frozen corpses (“ice anatomy”) and, based on this method, wrote “A Complete Course of Applied Anatomy of the Human Body” (1843-1848) and the atlas “Topographic Anatomy, Illustrated by Sections Made through a Frozen Body man in three directions" (1851-1859). These were the first manuals on topographic anatomy. All activities of N.I. Pirogov constituted an era in the development of medicine and anatomy. The operations and surgical techniques invented by Pirogov are still used in surgery today. Future doctors study using manuals, the beginning of which lies in the same works of Pirogov.
4. P.F. Lesgaft is the founder of the functional direction in anatomy.
P.F. Lesgaft (1837 - 1909) - the largest after N.I. Pirogov anatomist of Russia. He provided a number of evidence of the influence of the external environment and physical exercise on the structures of the body. Called for the study of living human anatomy. An outstanding researcher in the field of functional anatomy and the theory of physical education was P. F. Lesgaft (1837-1909), the author of the fundamental work “Fundamentals of Theoretical Anatomy.” P. F. Lesgaft is the founder of theoretical anatomy in Russia. He described the patterns of restructuring of bone substance under the influence of muscle traction, formulated the principles of the development of blood vessels and their relationships depending on the structure and function of organs, showed the importance of anastomoses between arteries in the blood supply to organs and parts of the body
Skeleton: definition, sources and stages of development, functional role.
The skeleton is a complex of dense formations that provide the shape of the body and organs, protection, delimitation, support and movement in space. Skeletal phylogeny: membranous stage, cartilaginous stage, bony stage. Ontogenesis of the skeleton: 4-5 weeks of fetal development - connective tissue stage of skeletal development; 6-7 weeks – cartilaginous stage; 7-8 weeks – bone tissue appears (bone stage). From the 8th week, the formation of primary (diaphyseal) ossification points begins. At the 9th month of uterine development, the formation of secondary points (epiphyseal) of ossification begins. Functions of the skeleton: ü Mechanical: supporting, protective, motor. ü Biological: participation in metabolism, hematopoietic.
Bone: definition, classification, structure.
Patterns of bone structure according to P.F. Lesgaft: ü Spongy substance is formed in places of greatest compression or tension. ü The degree of development of bone tissue is proportional to the activity of the muscles associated with a given bone. ü The tubular and arched structure of the bone provides the greatest strength with minimal consumption of bone material. ü The external shape of bones depends on the pressure on them from surrounding tissues and organs. ü Bone restructuring occurs under the influence of external forces. Classification of bones by development: primary, secondary. Classification of bones by form and function: tubular, spongy, flat, mixed, pneumatic, sesamoid. Bone as an organ consists of bone tissue (compact and spongy substance), covered on top with periosteum, and inside there is a medullary cavity.
Structure of bone tissue, definition of osteon.
Bone tissue is a type of connective tissue that consists of cellular elements (osteoblasts, osteocytes, osteoclasts) and intercellular substance. Types of bone tissue: ü Coarse fibrous. ü Lamellar. Osteon (Haversian system) is a system of bony plates concentrically located around a central canal containing blood vessels and nerves.
Osteogenesis, types of bone ossification.
Types of ossification a) primary Endesmal (occurs in the connective tissue of primary bones) b) secondary Perichondral (occurs on the outer surface of the cartilaginous rudiments of secondary bones with the participation of the perichondrium) Endochondral (occurs inside the cartilaginous rudiments of bones) Osteogenesis is the process of bone formation that occurs in three stage under the influence of special cells (osteoblasts).
Ontogenesis of the skull: sources of development, stages and timing.
The skull is a complex of bones connected by different types of joints that serve as support and protection for organs of various origins and functions. The skull is divided into 2 sections: the cerebral skull, the facial (visceral) skull. Ontogenesis of the skull: Development of the brain skull: mesenchyme of the sclerotomes of the cephalic somites around the cranial end of the notochord. For 1 month – membranous skull (primary bones are formed: frontal, parietal, squamosal and tympanic part of the temporal bone, squama of the occipital bone) At the beginning of 2 months. – cartilaginous base (parachordal and prechordal plates are formed, next to which cartilaginous capsules of the sensory organs are laid: olfactory, visual, auditory). At the end of 2 months. – convergence and fusion of cartilaginous plates and cartilaginous capsules, the formation of cartilaginous regions (secondary bones are formed): Ethmoidal region (fusion of nasal capsules and prechordal plates) → ethmoid bone and inferior nasal concha; Orbital region (fusion of optic capsules and prechordal plates) → most of the sphenoid bone; Labyrinthine region (fusion of the otic capsules and parachordal plates) → petrous part and mastoid process of the temporal bone; Occipital region (fusion of parachordal plates) → basilar, lateral parts and lower part of the squama of the occipital bone. Development of the facial skull: from the mesenchyme adjacent to the cranial part of the primary intestine, based on the visceral arches (a complex of three germ layers: ento-, ecto- and mesoderm), of which 5 pairs are formed, and between them 5 pairs of visceral pouches. From 1 visceral arch and the frontal process of the brain capsule - the upper and lower jaws and the auditory ossicles (malleus and incus). Of the 2 visceral arches - the stapes (auditory ossicle), the styloid process of the temporal bone, the lesser horns and part of the body of the hyoid bone. Of the 3 visceral arches - the large horns and part of the body of the hyoid bone. Of the 4 and 5 visceral arches - cartilage and muscles of the larynx, part of the anterior muscles of the neck. The remaining bones of the facial skull develop from primordia in the mesenchyme located on the sides and in front of the nasal capsules.
Variants, anomalies and malformations of the skull.
Anomalies of skull development: 1) acrania; 2) microcranium; 3) megacrania; 4) anomalies of jaw development - progeny, prognathia, macrognathia, micrognathia; 5) assimilation of the atlas by the occipital bone; 6) deformations of the bones of the facial skull - cleft palate, open nasolacrimal duct, cyclopia, etc. Malformations of the skull can manifest themselves in a discrepancy between the size of the skull and the volume of the brain and the presence of external deformities (craniosynostosis, hypertelorism); in incomplete closure of the bones of the skull and spinal canal with the formation of defects through which the contents of the skull and spinal canal can protrude (cerebral and spinal hernias); in deformation of the skull, leading to compression of important brain structures (platybasia, basilar impression). Craniostenosis (from the Greek kranion - skull + Greek stenosis - narrowing) is a congenital pathology of the development of the skull, manifested in the early fusion of cranial sutures, resulting in deformation of the skull and a discrepancy between its volume and the size of the brain.
Despite the differences in origin, general principles of development, the nature of morphological and functional disorders make it possible to consider clinical issues, diagnosis and treatment of most congenital deformities in one context. That is why, to study the pathogenesis of deformities, it is necessary to know the main stages of the formation of various parts of the skull and the factors influencing these processes.
From the point of view of simplicity of presentation expedient consider the growth and development of the facial and cerebral skulls separately, since the first is associated in its formation with the activity of the masticatory and facial muscles, the development of teeth and ENT organs, and the second with the growth of the brain.
First of all, we would like to briefly recall the general information about the processes bone formation. Accordingly, two variants of skeletal morphogenesis are possible - membranous and cartilaginous. Only in a very small group of bones are both pathways combined.
Phylogenetically, this is an older pathway of bone formation.
Development of bone structure, its transformation, occur throughout a person’s life and consist of processes of apposition and resorption of an exosteal and endosteal nature.
Brain skull, mainly formed by bones of direct connective tissue origin. Only the lower part of the body of the occipital bone and the lateral sections of the main bone have cartilaginous genesis. The first points of ossification in the mesenchymal premordial brain skull of the fetus appear in the 8th week of the intrauterine period, and by the birth of the child the integumentary bones of the skull have already been formed and the remains of connective tissue anlages are preserved only in the area of fontanelles, sutures and synchondroses.
Functional interactions of the skull and brain, which play a large role in the development of integumentary bones, appear already in the prenatal period. Before the birth of a child, his brain, gradually increasing in size, contributes to the acquisition of an almost regular rounded shape by the brain skull. During childbirth, the bones of the brain skull often shift in accordance with the direction of pressure and may partially retain their acquired position during the further growth of the child. The volume of the cerebral skull in a newborn sharply prevails over the volume of the facial skull (the skull of a newborn versus the volume of an adult).
During first 8 years of a child's life(mainly in the first months) the brain mass increases 8 times. This leads to a physiological long-term increase in intracranial pressure. The shape of a child's brain skull develops under pressure from the brain, blood and intracerebral fluid from the inside and developing muscles from the outside.
As the data shows clinical and radiological studies, the rapid growth of the integumentary bones leads to an increase in the parameters of the brain skull (height to the greatest extent and width to the least extent). The maximum increase in the size of the skull occurs in the first 1.5-2 years of a child’s life. In the formation of the brain skull, an important role is played by appositional bone formation and transformation processes due to the new formation of bone substance in the area of sutures.
Happens throughout life formation process in bones of the brain skull, which is carried out through resorptive processes on the inner surface of the bones, on the other hand, through the formation of new bone tissue on their outer surface.
Educational video on embryogenesis, ontogenesis of the skull - its growth and development
Table of contents of the topic “Anomalies in the development of the skull of children”:The development of the facial skull and brain skull should be considered separately, since they have independent embryonic rudiments, structural features and functions, although topographically they are in close relationships. In the construction of the brain skull, a more ancient formation takes part - the base of the skull, which goes through the cartilaginous stage of development, with which the capsules of the sensory organs and phylogenetically younger bones of the cranial vault and face, ossifying on the basis of membranous connective tissue, are associated. The base and vault of the skull take part in the formation of the bone container for the central nervous system and protect the brain from damage.
Development of the brain part of the skull. The bones of the base of the skull go through three stages of development: membranous, cartilaginous and osseous.
Primary segmentation in the head region of embryos is observed only in the occipital part, where at the level of the hindbrain an accumulation of mesenchyme appears around the notochord (Fig. 69). As the brain grows, the surrounding mesenchyme also develops; its deep layer serves as a derivative of the meninges, and the outer one turns into a membranous skull. The membranous skull in some aquatic animals persists throughout life, but in humans it is found only in the embryonic period and after birth in the form of fontanelles and layers of membranous tissue between the bones. During this period, the developing cerebral hemispheres do not encounter obstacles from the membranous skull.
69. Schematic diagram of precartilaginous accumulations of mesenchyme in a human embryo 9 mm long (according to Bardin).
1 - chord;
2 - occipital complex;
3 - III cervical vertebra;
4 - blade;
5 - hand bones;
6 - palmar plate;
7 - VII rib;
8 - I lumbar vertebra;
9 - pelvis;
10 - leg bones;
11 - sacral vertebrae.
70. Formation of the prechordal and perichordal plates of the developing skull.
1 - prechordal plates (crossbars);
2 - perichordal plates;
3 - chord;
4 - olfactory capsule;
5 - optic fossa;
6 - auditory capsule;
7 - main pharyngeal canal.
At the 7th week of intrauterine development, the transformation of the membranous tissue of the base of the skull into cartilaginous tissue is observed, while the roof and facial part remain membranous. The cartilaginous tissue of the base of the skull is divided into cranial crossbars lying in front of the chord - prechordal and at the edges of the chord - parachordal plates and capsules of the sensory organs (Fig. 70). During this period of development of the skull, blood vessels and nerves grow into its cartilaginous base and take part in the formation of future holes, crevices and canals of the bones of the base of the skull (Fig. 71. A, B). The cranial bars and parachordal plates fuse into a common plate, which has an opening in the place of the future sella turcica, located near the anterior end of the chord. Through this opening pass the cells of the posterior wall of the pharynx, forming the anterior lobe of the pituitary gland. The common cartilaginous plate also fuses with the olfactory, ocular and auditory capsules and with the membranous roof of the skull. The anterior end of the cartilaginous base of the skull is transformed into a vertical plate between the olfactory capsules in the form of the future nasal septum.
Later, at 8-10 weeks of intrauterine development, bone points appear in the cartilaginous base and roof of the membranous skull (see Development of individual bones of the skull).
71. Cartilaginous base of the skull (according to Hertwig).
A - embryo 7 weeks; B - fetus 3 months; 1 - olfactory capsule; 2 - ethmoid bone; 3 - superior orbital fissure; 4 - large wing of the sphenoid bone; 5 - sella turcica; 6 - torn hole; 7 - auditory capsule; 8 - jugular foramen; 9 - internal auditory opening; 10 - foramen magnum.
Development of the facial part of the skull. The development of facial bones must be considered and compared with the development and structure of the bones of aquatic animals. They retain the gill apparatus throughout their lives, while in the human embryo its rudiments exist for a relatively short time. In humans and mammals, during the development of the membranous base and cranial vault, seven gill arches are formed. During this period, the facial skull has many similarities with the skull of a shark (Fig. 72).
72. Shark skull (according to E. Gundrich).
1 - brain skull; 2 - opening for the exit of II, III, IV and V pairs of cranial nerves; 3 - palatoquadrate cartilage; 4 - Meckel's cartilage; 5 - infratemporal cartilage; 6 - hyoid cartilage; 7 - the hyoid cartilage itself; I - VII - gill arches.
The differences are that the shark has an open connection between the outer and inner gill pouches. In the human embryo, the gill slits are closed by connective tissue. Subsequently, various organs are formed from the gill arches (Table 2).
| Skull formations existing in the embryonic period in aquatic animals | Formations of the skull that exist in adult aquatic animals and in the embryonic period in humans | Derivation of gill arches in humans |
| I gill arch | Dorsal cartilage Ventral cartilage |
Incus (auditory ossicle) Lower jaw Hammer (auditory ossicle) |
| II gill arch | Hyoid-maxillary cartilage (upper part) Hyoid cartilage (lower part) | Stapes (auditory ossicle) Styloid process of the temporal bone, lesser horns of the hyoid bone, stylohyoid ligament |
| Cavity between I and II gill arches | Bryzgaltse | Tympanic cavity Eustachian tube |
| III gill arch | Gill arch Unpaired cartilage for connecting the gill arches |
Greater horns of the hyoid bone, body of the hyoid bone |
| IV branchial arch | Gill arch | Thyroid cartilage of the larynx |
| V branchial arch | » » | |
| VI branchial arch | Gill arches in aquatic animals | |
| VII gill arch | » » | Are being reduced |
Thus, only part of the bones of the facial skull (lower jaw, hyoid bone, auditory ossicles) develops from the branchial apparatus.
The process of formation of the facial skull can be traced in the human embryo and lower animal species. Using the example of the development of the skull, one can be convinced that man has gone through a complex path of evolutionary development from an aquatic ancestor to a terrestrial animal. Balfour and Dorn showed that the head represents a transformed anterior end of the body, which, before the development of the central nervous system, had the same structure as the whole body and was segmented. With the formation of the sense organs and the brain at the anterior end of the body and the corresponding transformation of the gill arches into the maxillary and submandibular arches, the vertebral sections of the notochordal part of the head fused with each other and provided the basis for the skull. Consequently, the prechordal and parachordal plates are transformed parts of the axial skeleton.
The bones of the skull begin to form in the early stages of prenatal ontogenesis, and very soon the first ossification centers appear (Table 15).
Frontal bone begins to form in the 9th week of embryogenesis from the first two ossification centers (future frontal tubercles) that arise in the connective tissue plate (membranous skull). In a newborn, the frontal bone consists of two halves connected by a median suture. The fusion of these two halves of the frontal bone occurs in the 2-7th year of a child’s life. The frontal sinus begins to develop in the 1st year of life (Table 16).
Most of sphenoid bone develops from cartilage, in which 5 pairs of ossification centers are formed at the 9th week of embryogenesis. Only the lateral portions of the greater wings and the medial plates of the pterygoid processes (with the exception of the pterygoid hook) develop from the connective tissue plate. The ossification centers gradually merge with each other. In a newborn, the sphenoid bone consists of three separate parts: the central part, which includes the body and lesser wings; large wings with lateral plates of pterygoid processes; medial plates of the pterygoid processes. These parts fuse into a single sphenoid bone during the 3-8th year of a child’s life. At the 3rd year, the sphenoid sinus begins to form in the body of this bone.
Occipital bone. The main and lateral parts of the occipital bone, as well as the lower part of the occipital squama, develop from cartilage, in which four separate ossification centers arise. The upper part of the occipital scales is formed from a connective tissue plate, in which two centers of ossification are formed. Ossification centers are formed at 8-10 weeks,
TABLE 14. Derivatives of the branchial arches in humans and the corresponding cranial nerves innervating them(by Braus)
|
ordinal number gill arches |
Arc name |
Derivatives of gill arches |
Cranial nerves innervating branchial arch derivatives |
|
1 gill or jaw |
Hammer, incus, upper and lower jaws |
Third branch of the trigeminal nerve (V) |
|
|
II branchial or hyoid |
Stapes, styloid process of the temporal bone, anterior part of the body and lesser horns of the hyoid bone stylohyoid ligament |
Facial nerve (VII) |
|
|
III gill |
Posterior part of the body and greater horns of the hyoid bone |
Glossopharyngeal nerve (IX) |
|
|
IV branchial |
Thyroid, cricoid, arytenoid, rizhkuvati and sphenoid cartilages of the larynx |
Superior laryngeal nerve - branch of the vagus nerve (X) |
|
|
V branchial |
The above listed cartilages of the larynx |
Recurrent laryngeal nerve - branch of the vagus nerve (X) |
Rice. 83. Position of gill (IV) arches and their derivatives (diagram, modification by A. Bystrov)
and their fusion into one occipital bone occurs only after birth in the 3-5th year of a child’s life.
Parietal bone develops from a connective tissue membrane, the primary ossification center appears at the site of the future parietal tubercle at the 8th week of intrauterine life.
Ethmoid bone is formed from 3 ossification centers: the median and two lateral, arising in the cartilage of the nasal capsule. A perpendicular plate develops from the median center of ossification, and ethmoid labyrinths develop from the lateral ones.
The joining of parts into a single ethmoid bone occurs after birth in the 6th year of a child’s life.
Temporal bone begins to form in the 5th-6th month of intrauterine life after the appearance of ossification centers in the cartilaginous auditory capsule (the future stony part). Only the squamous part of the temporal bone develops from connective tissue; the center of ossification appears in it at the 9th week. In the tympanic part, the center of ossification appears at the 10th week of prenatal ontogenesis. The styloid process develops from the cartilage of the second visceral arch from two centers of ossification - one arises in front
TABLE 15. Timing of formation of ossification points in the bones of the human skull (according to Petten)
|
The names of the skull bones and their parts |
Timing of formation of ossification points (months of intrauterine development) |
merger timing ossification points |
|
frontal bone |
||
|
Sphenoid bone: wings |
4 months intrauterine development -1 year |
|
|
front part of the body |
||
|
back of the body |
||
|
side plates |
||
|
Occipital bone: |
||
|
upper part of the scales |
||
|
main part |
||
|
side parts |
||
|
Temporal bone: |
9 months intrauterine development -1 year |
|
|
scaly part |
||
|
drum part |
||
|
rocky part |
||
|
parietal bone |
||
|
Upper jaw |
||
|
Lower jaw: |
||
|
chin protuberance |
||
|
cheekbone |
||
|
ethmoid bone |
6 months intrauterine development - 4 years |
|
|
nose braid |
||
|
lacrimal bone |
||
|
palatine bone |
||
|
Hyoid bone: |
||
|
big horns |
||
|
small horns |
||
|
Auditory ossicles: |
||
|
hammer |
||
|
anvil |
||
birth, and the second in the 2nd year of the child’s life. The fusion of parts of the temporal bone begins before the child is born and continues until the age of 13. The styloid process grows onto the temporal bone in the 2-12th year of a child’s life.
Upper jaw is formed by the fusion of several ossification centers that arise at the end of the 2nd month of embryogenesis in the connective tissue of the maxillary and middle nasal (frontal) processes. One center of ossification is formed in the future alveolar process at the level of the dental cells for the incisors, from which the incisive bone is formed in the prenatal period. The incisive bone grows into the upper jaw after birth. The maxillary sinus begins to form in the 5-6th month of intrauterine life.
Small bones of the facial skull (palatine, nasal, lacrimal, zygomatic bones And ploughshare) develop from ossification centers that arise in the membranous skull at the end of the 2nd - beginning of the 3rd month of intrauterine life. Inferior turbinate And ethmoid bone develop from the cartilage of the nasal capsule.
Lower jaw develops from a connective tissue plate surrounding Meckel's cartilage and initially consists of two halves. In each half of the membranous mandible at the 2nd month EMS
TABLE 16. Timing of formation of cavities (sinuses and cells) in the air-bearing bones of the skull
During riogenesis, several ossification centers are formed. These ossification centers gradually fuse with each other. Both halves of the lower jaw fuse into one bone after birth only in the 1st or 2nd year of the child’s life. In a child before teething, the angle of the lower jaw is obtuse, its branches are short and inclined back. In 20-40 year old people, the angle of the lower jaw approaches straight, its branches are located vertically. In old people whose teeth have fallen out, the angle of the lower jaw becomes obtuse, the length of the branches decreases, and the cellular part atrophies.
Hyoid bone formed from the cartilage of the II gill arch (small horns) and III gill arch (body and large horns). Ossification centers in the cartilage of the body and large horns of the hyoid bone appear before birth (8-10 months), and in small angles - in the 1st-2nd year of the child’s life. The fusion of these parts into one hyoid bone occurs only in the 25-30th year of a person’s life.
7. Development of the cerebral part of the skull. Variants and anomalies of its development.
Brain section of the skull develops from the mesenchyme surrounding the rapidly growing brain. The mesenchymal cover turns into a connective tissue membrane - the stage of the membranous skull. In the area of the arch, this shell is subsequently replaced by bone. Cartilaginous tissue appears only at the base of the skull, near the anterior section of the notochord, which ends dorsal to the pharynx, posterior to the future pituitary stalk. The areas of cartilage lying next to the notochord are called perichordal (parachordal) cartilages, and in front of the notochord are the prechordal plates and cranial crossbars. Subsequently, the cartilage at the base of the skull is replaced by bone, with the exception of small areas (synchondrosis), which persist in adults until a certain age.
Thus, in humans, the vault (roof) of the skull goes through two stages in its development: membranous (connective tissue) and bone, and the base of the skull goes through three stages: membranous, cartilaginous and bone.
Frontal bone. In approximately 10% of cases, the frontal bone consists of two parts, with a frontal suture remaining between them, sutura frontlis (sutura metopica). The size of the frontal sinus varies, very rarely the sinus is absent.
Sphenoid bone. Failure of fusion of the anterior and posterior halves of the body of the sphenoid bone leads to the formation of a narrow, so-called craniopharyngeal canal, in the center of the sella turcica. The foramen ovale and foramen spinosum sometimes merge into one common foramen; the foramen spinosum may be absent.
Occipital bone. The upper part of the occipital squama, in whole or in part, can be separated from the rest of the occipital bone by a transverse suture. As a result, a special triangular bone is identified - the interparietal bone, os in- terparietdle.
Ethmoid bone. The shape and size of the cells of the ethmoid bone are very variable. The highest nasal concha is often found, concha nasdlis suprema.
Parietal bone. Due to the fact that the ossification points do not merge, each parietal bone can consist of an upper and lower halves.
Temporal bone. The jugular notch of the temporal bone can be divided into two parts by the interjugular process. If there is the same process in the jugular notch of the occipital bone, a double jugular foramen is formed. The styloid process of the temporal bone may be absent, but more often it is long, and can even reach the hyoid bone in the case of ossification of the stylohyoid ligament.
8. Development of the facial part of the skull. The first and second visceral arches, their derivatives.
Facial part of the skull develops from the mesenchyme adjacent to the initial part of the primary intestine.
The facial part of the skull develops from the mesenchyme adjacent to the initial part of the primary intestine. In the mesenchyme between the gill pouches, cartilaginous gill arches are formed. The first two of them are of particular importance - the visceral arches, on the basis of which the visceral skull develops.
First visceral arch (maxillary) in humans, it gives rise to two auditory ossicles (the malleus and the incus) and the so-called Meckel’s cartilage, on the basis of which the lower jaw develops from the mesenchyme.
Second visceral arch (hyoid) consists of two parts - upper and lower. From the upper part develop the auditory ossicle - the stapes and the styloid process of the temporal bone. The lower part goes to the formation of the small horns of the hyoid bone. The large horns and body of the hyoid bone are formed from the third visceral (I branchial) arch.
Thus, on the basis of the visceral arches from the connective tissue, the small bones of the facial part of the skull and the lower jaw develop.