Isceral Skeleton

or cranial skeleton, in vertebrate animals and man the skeletal elements formed in the oral and pharyngeal regions of the digestive tube. In this location lower vertebrates have gill clefts separated by interbranchial septa in which visceral support elements, or gill arches, arise. In the ancestors of the vertebrates (according to A. N. Severtsov) the gill clefts originated directly behind the mouth. There were as many as 17 of them. The anterior and posterior gill clefts and visceral arches disappeared as the vertebrates evolved. The visceral skeleton evolved in two directions. In agnathous organisms (fossil shelled organisms and recent cyclostomes) the visceral arches are intact and situated exteriorly to the gill pouches. In the lamprey they are interconnected longitudinally and form a flexible branchial network. The anterior gill arches form the suborbital arch of the skull and lateral cartilages of oral suckers. In gnathostomes the gill leaflets develop exteriorly to the skeleton. The gill arches are divided into four loosely connected components. The mobility of the gill arches facilitates the respiratory function of the gills and also to some extent the holding of food in the oral cavity. Thus the respiratory function of the anterior gill arches has been lost. The first two atrophied and their residua are present in lower fishes as labial cartilages. The third gill arch evolved into an organ for the active grasping of food; it evolved into jaws and formed the primary maxilla (palatoquadrate cartilage) and primary mandible (Meckel’s cartilage). The fourth gill arch forms the hyoid arch, which consists of an upper appendage, which in most fish connects the upper jaw to the skull, and a lower appendage, the hyoid cartilage proper. The successive visceral arches form the gill arches proper, usually five but sometimes six or seven in number.

In bony fishes the labial cartilages disappear from the visceral skeleton and ossifications develop on the palatoquad-rate cartilage, with the palatine bone forming on the anterior end, the quadrate bone on the posterior end, and the pterygoid bones between them. The visceral skeleton of bony fishes went through a significant change with the appearance of secondary jaws that originated from the flat bones. The premaxillary bone and the maxilla form the upper secondary jaw. The lower secondary jaw is formed by the dentary, which embraces the anterior half of Meckel’s cartilage. The posterior half of Meckel’s cartilage ossifies as an independent articular process that forms the mandibular joint together with the quadrate bone. Secondary bones, such as the angular and supra-angular, are also formed here. The bony gill cover develops in bony fishes on the hyoid arch. The appendage is divided into the appendage proper and connecting bone, which increases the mobility of the mandibular apparatus. The hyoid ossifies. There are always five gill arches.

The primary upper jaw fuses with the skull in all terrestrial vertebrates to form the bony parts of the palate (autostyl). The premaxillary bone and the maxilla function as jaws. The lower jaw in terrestrial vertebrates other than mammals consists of the same bones as in bony fishes; the mandibular joint is formed by the quadrate bone and the articular. The primary function of the appendage changes from an “appendage” to an organ that transmits sound oscillations from the tympanic membrane to the inner ear, and the structure be-comes an auditory ossicle (columella) situated in the cavity of the middle ear. The hyoid and gill arches atrophy and, after fusion, form the hyoid bone with its processes. In mammals the mandible consists exclusively of a dentary that articulates with the temporal bone. This secondary mandibular joint re-places the primary joint between the quadrate bone and the articular, which disappears. These bones in mammals are found in the cavity of the middle ear, where they form the auditory ossicles; the quadrate bone forms the incus and the articular, the maleus. The stapes in mammals arises from the auditory ossicle (columella).

In mammals the tympanic bone develops into the angular bone of the lower jaw of their ancestors. The body of the hyoid bone and its anterior cornua originate from the hyoid arch, and the posterior cornua originate from the gill arch. The second and third gill arches form the thyroid cartilage of the larynx. The epiglottis is formed from the fourth arch, and the arytenoid and, according to some data, tracheal cartilages originate from the fifth arch.