328 



MAN, A MAMMAL 



The skull : F., frontal bone; P., parietal bone ; 

 T., temporal bone ; SP., sphenoid bone ; O., occi- 

 pital bone; U.J., superior maxillary (upper 

 jaw) bone ; L.J., inferior maxillary (lower jaw) 

 bone. 



The Human Skull. — The skull stows wonderful adaptations for its 

 varied functions. The brain case is compactly built, its arched roof 

 giving strength. The eye and inner ear are protected in sockets of bone. 



The lower jaw works upon a 

 hinge, and furnishes attach- 

 ment for strong muscles which 

 move the jaw. 



The skeleton, besides the 

 purposes already described, 

 protects certain organs in 

 the body cavity of man. 



Other Organs. ^ — We 

 have seen that a body 

 cavity has developed in aU 

 animals which are more 

 complex than the baglike 

 hydra, and that a food tube 

 has come to lie within this 

 space. In all such animals 

 the structures which have to do with digestion and absorption of food, 

 most of the structures which have to do with the circulation of this 

 food and of the blood, and organs which give oxygen to the blood, 

 as well as the organs of excretion and of reproduction, he within 

 the body cavity. These organs we shall discuss in detail later. 



Nerves. — Other structures, known as nerves, are found in prac- 

 tically all parts of the body. We find that nerves have their end- 

 ings in the skin, in muscle, and in the cells of glands in various parts 

 of the body; we find a nerve supply to the heart, lungs, and 

 other structures within the body cavity. The most important 

 part of the nervous system in vertebrate animals lies within the 

 cavity formed by bones making up the skull and the vertebral 

 column. This central nervous system, the spinal column and 

 the bram, is a characteristic of the vertebrate animals. 



General Functions of the Nervous System. — We have seen that, 

 in the simplest of animals, one cell performs the functions neces- 

 sary to its existence. In the more complex animals, where groups 

 of cells form tissues, each having a different function, a nervous 

 system is developed. The functions of the human nervous system are : 



