Visceral skeleton of bony fishes. The meaning of the visceral skeleton in the Great Soviet Encyclopedia, BSE

Skeleton The dog's skeleton performs a supporting function and also protects internal organs from external damage. It is formed by bones, cartilage and ligaments. In the skeleton of a dog there are from 271 to 282 bones, connected to each other continuously (fused) or discontinuously (with

Vertebrates. The vertebrate skeleton is formed not only by bones: it includes cartilage and connective tissue, and sometimes it includes various skin formations.

In vertebrates, it is customary to distinguish the axial skeleton (skull, notochord, spine, ribs) and the skeleton of the limbs, including their girdles (shoulder and pelvic) and free sections.

Skull (cranium) – the skeleton of the head of vertebrates. There are cerebral skull (craniumcelebrale, s. neurocranium) and visceral (craniumviscerale, s. splanchnocranium).

During the process of phylogenesis, the angle between the longitudinal axis of the brain skull and the facial axis increases.

The skull determines the shape of the head. The cranium forms the container for the brain, organs of smell, vision, balance and hearing. The bones of the facial skull form the bony basis for the initial part of the digestive and respiratory systems (oral cavity and nasal cavity).

Integumentary bones arise as calcareous deposits in the dermal layer of the skin. In some ancient fish, they were plates of shell that protected the brain, cranial nerves and sensory organs located on the head. In all higher forms, these plates migrated into the depths, were incorporated into the original cartilaginous skull and formed new bones, closely related to the replacement ones. Almost all of the outer bones of the skull come from the dermal layer of the skin.

The visceral elements of the skull are derivatives of the cartilaginous gill arches that arose in the walls of the pharynx during the development of gills in vertebrates. In fish, the first two arches have changed and turned into the jaw and hyoid apparatus. In typical cases, they retain 5 more gill arches, but in some genera their number has decreased. The primitive modern shark sevengill ( Heptanchus) behind the maxillary and hyoid arches there are as many as seven gill arches. In bony fishes, the jaw cartilages are lined with numerous integumentary bones; the latter also form gill covers that protect the delicate gill filaments. During the evolution of vertebrates, the original jaw cartilages were steadily reduced until they disappeared completely. If in crocodiles the remainder of the original cartilage in the lower jaw is lined with 5 paired integumentary bones, then in mammals only one of them remains - the tooth, which completely forms the skeleton of the lower jaw.

The skull of ancient amphibians contained heavy integumentary plates and was similar in this respect to the typical skull of lobe-finned fish. In modern amphibians, both applique and replacement bones are greatly reduced. There are fewer of them in the skull of frogs and salamanders than in other vertebrates with a bony skeleton, and in the latter group many elements remain cartilaginous. In turtles and crocodiles, the skull bones are numerous and tightly fused to each other. In lizards and snakes they are relatively small, with the external elements separated by wide intervals, as in frogs or toads. In snakes, the right and left branches of the lower jaw are very loosely connected to each other and to the cranium by elastic ligaments, which allows these reptiles to swallow relatively large prey. In birds, the skull bones are thin but very hard; in adults they have fused so completely that several sutures have disappeared. The orbital sockets are very large; the roof of the relatively huge braincase is formed by thin integumentary bones; the light jaws are covered with horny sheaths. In mammals, the skull is heavy and includes powerful jaws with teeth. The remains of the cartilaginous jaws moved to the middle ear and formed its bones - the hammer and the incus.

Comparative anatomy

The brain and facial skulls have different phylogenetic origins. The brain skull is a continuation of the axial skeleton of the body. In lower vertebrates, it is built on the cartilages that form the braincase, ear and nasal capsules. The braincase consists of the chordal (posterior) and prechordal (anterior) parts, the border between them is the sella turcica. The notochordal part develops from the cephalic sclerotomes and has signs of a segmental structure; the occipital and auricular regions are distinguished in it. The prechordal part is unsegmented and is divided into the orbital and nasal regions. Evolutionary transformations of the cerebral skull are determined, first of all, by the development of the brain and sensory organs.

The facial skull in agnathans is represented by several pairs of gill arches, metamerically located in the walls of the foregut. In fish, the anterior gill arches are transformed into jaws, and in terrestrial vertebrates, auditory ossicles and the hyoid apparatus also develop from them. The primary (primordial) cartilaginous skull is most developed in cartilaginous fish. In ancient lungfishes, bones appear at the base of the skull, replacing cartilage, and integumentary bones are formed in the cranial vault as a result of the fusion of skin scales. The visceral skull of bony fishes consists of a larger number of small replacement and integumentary bones. With the transition to a terrestrial lifestyle, the total number of skull bones decreases, some of them merge with each other, and some disappear. The way the jaws are attached to the skull changes. In reptiles, a secondary bony palate is formed, separating the nasal cavity from the oral cavity, and temporal pits and temporal arches are formed. Fossil theriodont reptiles have a head skeleton similar to that of mammals.

In mammals, for the first time, a joint is formed between the lower jaw and the temporal bone, the muscle relief of the bones is smoothed, the brow ridges are reduced, the jaws are shortened, the alveolar processes are reduced, and the external nose and chin protrusion are formed.

During the evolution of vertebrates, the visceral skeleton undergoes great changes; in origin it is associated with the gill-respiratory function of aquatic vertebrates. This is reflected in the embryonic development of higher vertebrates and humans. In their embryos, in the early stages of embryonic development, the rudiments of gill openings appear, between which blood vessels, muscles and elements of the visceral skeleton are laid, forming the jaw, hyoid and gill apparatus. The study of the visceral skeleton played a major role in the development of comparative anatomy of vertebrates.

In lower aquatic vertebrates, the visceral skeleton consists of a successive row of identical paired visceral arches located in the walls of the oral and pharyngeal region of the digestive tube on the right and left between the gill slits. They serve as the skeletal elements of aquatic respiration organs - gills, for example, in lancelets and cyclostomes.

In all fish and terrestrial vertebrates, the three anterior visceral arches acquired the function of capturing food, processing it and swallowing it (i.e., they formed the basis of the jaw and pharyngeal apparatus). The jaw arch consists of the upper and lower jaws, the hyoid arch serves as a suspension when combining the jaw apparatus with the brain skull. The remaining visceral arches are each divided into four elements and form the gill apparatus.

In terrestrial vertebrates, due to the transition to air respiration, the gill apparatus is gradually reduced. The primary upper jaw - the palatoquadrate cartilage - grows to the bottom of the skull and fuses with the secondary dermal bones. The lower jaw is attached to the bottom of the skull through the quadrate bone. The upper element of the hyoid arch moves to the middle ear and turns into an auditory bone - the stapes; the lower elements of the hyoid arch turn into the hyoid apparatus, and the system of branchial arches is reduced. In reptiles and birds, a movable articulation of the upper jaw with the skull (kineticism) is formed in the jaw apparatus, which is an adaptation to various methods of grasping food. In mammals and humans, kineticism disappears, but a movable block attachment of the lower jaw to the skull through the condylar process develops, and in the middle ear, due to the elements of the visceral skeleton, a system of three auditory ossicles (hammer, incus and stapes) is formed. The formation of a movable articulation of the lower jaw with the skull makes it possible to mechanically process food in the oral cavity; Various types of food chewing are formed - circular, transverse, longitudinal.

Evolution of the visceral skeleton.

In lower fish (cartilaginous fish), the visceral skull consists of a number of cartilaginous visceral arches covering the pharynx like a hoop, of which the 1st (maxillary) arch consists of only two large cartilages, elongated in the anteroposterior direction - the upper (palatoquadrate) and the lower (Meckelian) . The upper and lower cartilages of each side are fused to each other and perform the functions of the jaws (primary jaws). The 2nd visceral arch consists of two paired and one unpaired cartilage, connecting the paired cartilages with each other from below. The upper element of the pair, the larger one, is the hyomandibular cartilage, the lower paired element is the hyoid, and the unpaired element is the copula. The upper edge of the hyomandibular cartilage is connected to the cranium, the lower edge is connected to the hyoid cartilage, and the anterior edge is connected to the jaw arch lying in front. Thus, the hyomandibular cartilage acts as a suspension for the jaw arch; it is attached to the skull using the hyoid arch. This type of connection of the jaws with the skull is called hyostyly (hyostyle skull) and is characteristic of lower vertebrates. The remaining arches (3-7) form a support for the respiratory apparatus.

In higher fishes (bony fish), the main changes in the visceral skull concern the jaw arch. The upper jaw, instead of one large palatine quadrate cartilage, consists of 5 elements - the palatine cartilage, the quadrate bone and 3 pterygoid bones. In front of the primary upper jaw, 2 large overhead bones are formed - the premaxillary and maxillary, equipped with large teeth, which become the secondary upper jaws. The distal end of the primary mandible is also covered by the large dentary, which projects far forward and forms the secondary mandible. Thus, the function of the jaws in higher fish passes to secondary jaws formed by superimposed bones. The hyoid arch retains its previous function of suspending the jaws from the skull. Consequently, the skull of higher fishes is also hyostyle.

In amphibians, significant changes concern mainly the visceral region, since with the transition to a terrestrial lifestyle, gill respiration is replaced by cutaneous-pulmonary respiration.

In relation to the visceral skull, one of the main differences is the new way in which the jaw arch connects to the skull. Amphibians, in contrast to the hyostylous skull of fish, have an autostyle skull, i.e., their jawbone is connected to the skull directly, without the help of the hyoid arch, due to the fusion of the palatine cartilage of the jaw arch (primary upper jaw) along its entire length with the axial skull. The mandibular section articulates with the maxillary section and thus also receives a connection with the skull without the help of the hyoid arch. Thanks to this, the hyomandibular cartilage is freed from its function of suspending the jaws.

The new method of attaching the jaws to the skull entails a number of important changes in the hyoid arch. The hyomandibular cartilage is greatly reduced and acquires a completely new function - it is part of the auditory apparatus as an auditory ossicle (column). The change in functions was facilitated by the location of the hyomandibular cartilage next to the wall of the auditory capsule and the transformation of the first visceral fissure, in which the reduced hyomandibular cartilage was located, into the air cavity of the middle ear. Once inside the tympanic cavity, the hyomandibular cartilage becomes the auditory ossicle, which is involved in the transmission of sound vibrations to the inner ear. This was important when amphibians mastered the terrestrial environment, since it provided a more perfect perception of sound signals, which in terrestrial conditions are weaker than in the aquatic environment. Partially the hyoid arch - the hyoid cartilage, together with the branchial arches, forms a support for the tongue and the hyoid apparatus, partially - the laryngeal cartilages.

In reptile embryos, four pairs of gill arches and gill slits are also formed, of which only one breaks out, namely the first, located between the maxillary and hyoid arches, while the rest quickly disappear. The visceral skull of reptiles, like amphibians, is autostyle. However, there are some differences. The anterior element of the primary upper jaw, the palatine cartilage, is reduced. Therefore, only the posterior part, the quadrate bone, is involved in the attachment of the upper jaw to the skull. Accordingly, the surface area of ​​the attachment decreases. The lower jaw is connected to the quadrate bone of the upper jaw and is thus attached to the skull. The single gill slit, which breaks out during the embryonic period, is transformed into the cavity of the middle ear, and the hyomandibular cartilage becomes the auditory ossicle. The rest of the visceral skeleton forms the hyoid apparatus, which consists of the body of the hyoid bone and three pairs of processes. The body of the hyoid bone is formed by the fusion of the copulae of the hyoid arch and all the branchial arches. The anterior horns of this bone correspond to the lower paired element of the hyoid arch - the hyoid, and the posterior horns - to the paired elements of the first two branchial arches.

The main feature of the visceral skull of mammals is the appearance of a fundamentally new type of articulation of the lower jaw with the skull, namely, the lower jaw is attached to the skull directly, forming a movable joint with the squamosal bone of the skull. This articulation involves only the distal portion of the integumentary dentary (secondary mandible). Its posterior end in mammals is curved upward and ends with an articular process. Due to the formation of this joint, the quadrate bone of the primary upper jaw loses its function as a suspension of the lower jaw and turns into an auditory bone, which is called the incus. During embryonic development, the primary lower jaw completely leaves the lower jaw and is also transformed into an auditory bone, which is called the malleus. And finally, the upper part of the hyoid arch - a homologue of the hyomandibular cartilage - is transformed into the third auditory ossicle - the stapes. Thus, in mammals, instead of one, three auditory ossicles are formed, which form a functionally single chain.

The lower part of the hyoid arch in mammals is transformed into the anterior horns of the hyoid bone. The first branchial arch gives rise to the posterior horns, and its copula gives rise to the body of the hyoid bone; The 2nd and 3rd branchial arches form the thyroid cartilage, which appears for the first time in the process of evolution in mammals, and the 4th and 5th branchial arches provide material for the remaining laryngeal cartilages, and also, possibly, for the tracheal ones.