432 MAN BECOMES THE CONQUEROR OF THE WORLD 



A reflex arc. Explain this diagram. 



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 

 ■cell bocix the source of an- 



noyance when 

 tickled with a 

 feather, are exam- 

 ples of reflexes. 

 In a reflex act, a 

 person does not 

 think before act- 

 ing. The nervous 

 impulse comes 

 from the outside 

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

 the lower part of the brain. The message is short-circuited back 

 to the surface by motor nerves, without ever having reached the 

 thinking centers. 



Practical Exercise 3. Make a list of all the reflex acts that you have made 

 during the past twenty-four hours. Approximately what proportion of your 

 actions are reflexes ? 



The brain of man. In man, the central nervous system con- 

 sists of a brain and spinal cord inclosed 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 

 cerebrum makes up the largest part. In this respect it differs 

 from the cerebrum 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 shows many convolutions or folds which give 

 a large surface. The cell bodies and synapses of the neurons are 

 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 nerve fibers which pass to 

 other parts of the brain and down into the spinal cord. Below 

 the cerebrum lies a smaller portion of the brain, the cerebellum. 

 The two sides of the cerebellum are connected by a band of nerve 



