VISCERAL SKELETON In the lower vertebrates as well as in the embryo of man, a number of cartilaginous or bony arches encircle the mouth and pharynx (anterior part of the food tube), just as hoops encircle a barrel. There is little doubt that, when they first appeared in the history of evolution, all these bars supported gills and bounded gill slits, but in all existing types the first arch has been modified to surround the mouth and to act as both upper and lower jaws, gaining in different animals a more or less complete con nection with the cranium or brain-containing part of the skull. The first of these visceral arches, therefore, is known as the oral or jaw arch and, as has been shown, the muscles in connection with it are supplied by the fifth nerve (see MUSCULAR SYSTEM; and NERVE: Cranial). The second visceral arch is the hyoid and is accompanied by the seventh or facial nerve. The third visceral or first branchial arch of most writers has the ninth or glosso pharyngeal for its nerve supply, while the arches behind this are supplied by the vagus or tenth nerve.
In man the maxilla, palate, internal pterygoid plate, malar and tympanic bones as well as the ear ossicles, mandible, hyoid bone and thyroid cartilage are developed in connection with this vis ceral skeleton. Of these the ear ossicles are described in the article EAR, the thyroid cartilage in that on the RESPIRATORY SYSTEM, while the other bones, with the exception of the hyoid, are treated under SKULL. It therefore only remains to describe here the hyoid bone of man.
The body (basihyal) is rectangular with its long axis horizontal; behind it is concave from above downward and from side to side. In front it attaches several muscles, but behind it is smooth and is separated from the thyrohyoid membrane by a bursa. From its upper border this membrane runs downward to the thyroid carti lage. The great cornua (thyrohyals) are attached to each side of the body by cartilage until middle life and afterwards by bony union. They curve upward and backward round the side of the
pharynx and are laterally compressed. To their inner surfaces the thyrohyoid membrane is attached, while their knob-like ends are connected with the superior cornua of the thyroid cartilage by the lateral thyrohyoid ligaments.
The small cornua (ceratohyals) are about a quarter of an inch long. It is only in late life that they become united with the body by bony union, if they ever do so. At their apices they are connected with the tips of the styloid processes by the long stylo hyoid ligaments (epihyals).
In each of these processes carti lage is formed in the lower verte brates, which in the case of the mandible (lower jaw) reaches to the mid-ventral line and forms what is known as Meckel's carti lage; but in the maxillary pro cess the stage of chondrification is suppressed in man and other mammals, and the palato-quad rate cartilaginous bar which is so evident in embryo fishes and amphibians is not formed. Thus both the maxillary and the mandibular bars are derivatives of the first visceral arch. In the maxillary process a membrane bone is formed which blends with the sphenoid to form the internal pterygoid plate, while in front (ventrad) of this the upper jaw (maxilla) is developed in membrane by several centres. Of these, one, or perhaps two, form the premaxilla, each of the latter con tributing a socket for one of the two incisor teeth. When these premaxillary sutures fail to unite, the deformity known as "cleft palate" is produced and this may occur either between the lateral incisor and the canine or between the central and lateral incisor teeth.