NERVES AND BRAIN OF MAN 



199 



Tothebrd]in 



of muscles. These are called motor nerves. Still other neurons 

 connect the sensory and motor neurons. These are called asso- 

 ciative neurons. 



Reflexes and their Place in Our Lives. — We have seen that 

 reflexes play a very important part in the responsive life of simple 

 animals. They are equally important in our own lives. The 

 involuntary brushing of a fly from the face, or the attempt to 

 move away from the source of annoyance when tickled with a 

 feather, are examples of reflexes. In a reflex act, a person does 

 not think before acting. The nervous impulse comes from the 

 outside sensory cells to motor cells in the spinal cord, or in the 

 cerebel'lum, the lower part 

 of the brain. The mes- 

 sage is short-circuited 

 back to the surface by 

 motor nerves, without 

 ever having reached the 

 thinking centers. 



The Brain of Man. — 

 In man, the central ner- 

 vous system consists of a 

 hrain and spinal cord in- 

 closed in a bony case. 

 From the brain, twelve 

 pairs of nerves are given off; thirty-one pairs more leave the 

 spinal cord. The brain has three divisions. The cer'ebrum makes 

 up the largest part. In this respect it differs from the cere- 

 brum of the frog and lower vertebrates. It is divided into two 

 lobes, the hemispheres, which are connected with each other by a 

 broad band of nerve fibers. The outer surface of the cerebrum is 

 gray. It is thrown into folds or convolutions which give a large 

 surface, the cell bodies of the neurons being found in this part of 

 the cerebrum. Holding the cell bodies and fibers in place is a kind 

 of connective tissue. The inner part (white in color) is composed 

 largely of fibers which pass to other parts of the brain and down 

 into the spinal cord. Under the cerebrum lies the little brain, or 

 cerebellum. The two sides of the cerebellum are connected by a 

 band of nerve fibers which run around into the lower hind brain or 



Diagram of the nerve path of a simple reflex 

 action. 



