THE SKELETAL SYSTEM 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 modi- 

 fied 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 prob- 

 able 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 '* His- 

 tory 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 hones. 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 hones. 



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 classi- 

 fication of the races of man. The irregular lines on the 



