Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/21

7 and the inner and middle ears. It offers some protection to the eyes also. The ventral part is modified to form the upper and lower jaws which are used in grasping, killing, and chewing the food. The skull may be divided into two parts, the cranium, which surrounds the brain, and the visceral skeleton. The latter includes some of the bones of the face, of the upper and lower jaws, some of the bones on the lateral surface of the head in the ear region, and the small sound transmitting bones of the middle ear. The visceral skeleton of the rat is doubtless a highly modified heritage from the fish-like ancestors of the mammals. The chief function of the visceral skeleton of the fishes is to support the gills and lend strength to the jaws. The student should familiarize himself, either through works on evolution, or with the aid of the instructor, with the probable changes by which the visceral skeleton, epiglottis, and the laryngeal and tracheal cartilages of the mammals have evolved from the visceral arches of fishes. Wilder's "History of the Human Body" discusses this question.

The parts of the skull develop in two ways. The floor, and a portion of the sides and back of the cranium consist of bones which have developed from centers of ossification in the embryonic cartilage of these regions. These are the cartilage bones. The roof of the cranium, the remainder of the sides, the hard palate, and the bones of both jaws are intramembranous in origin. They are therefore called membrane bones.

Dorsal aspect of skull. The skull is long and narrow, gradually decreasing in width from the posterior to the anterior end. The width of the human skull is relatively greater, as compared with the length, than the width of the rat skull. The ratio of cranial width to length, or the cephalic index, is an important characteristic in the classification of the races of man. The irregular lines on the