336 ZOOLOGY 



of the body. These nerves are different in function ; 

 afferent or sensory nerves convey impulses to the brain 

 or cord ; efferent or motor nerves convey them in the re- 

 verse direction, and are the means whereby muscular 

 activity is stimulated. 



Primitively the brain is a swelling at the anterior end 

 of the spinal cord, and in the course of evolution it be- 

 comes divided into three vesicles known as the fore-, 

 mid-, and hind-brain. These vesicles are hollow, and 

 the cavities become variously modified. The fore- 

 brain gives rise to the cerebral hemispheres, which in 

 man occupy most of the surface of the brain. Anteri- 

 orly the olfactory lobes, connected with the sense of smell, 

 are developed. The upper and side parts of the mid- 

 brain form the optic lobes, having to do with the sense of 

 sight. The hind-brain forms the cerebellum (little brain) 

 anteriorly and the medulla oblongata posteriorly the 

 latter directly continuous with the spinal cord. If we 

 take any one of the lower vertebrates, such as a fish or 

 a frog, we find the organs of immediate sensation well 

 developed, but that part of the brain which keeps the 

 record of past experiences is very small. In man, on 

 the other hand, the part connected with memory and 

 reflection is very large. Thus the lower vertebrates act 

 almost wholly in response to stimuli just received, 

 whereas man's actions depend on past as well as present 

 experiences. It is possible to predict exactly what a 

 fish will do under given circumstances, almost as though 

 it were a mere machine. One cannot make similar pre- 

 dictions about a man, except in the case of actions still 

 brought about by reflexes which are not under the con- 

 trol of the brain. Such reflexes are observed in tickling, 

 which may produce irresistible kicking or coughing ac- 

 cording to the part stimulated. Others, like those pro- 



