THE STORY OF THE BRAIN AND NERVE 115 



frog and man the independence of the spinal cord as 

 a series of nerve centres is thus demonstrated, for 

 in the headless frog the control of its movements 

 can only be effected through the nerve cells of the 

 spinal cord which so far govern its body in the 

 absence of its brain, and in man the impression made 

 upon the feet passing up the cord is dealt with by 

 the nerve cells below the break, which thus "reflect" 

 the message to the muscles, and so produce the 

 movement of the legs. Research has shown that in 

 the upper part of the spinal cord there are nerve 

 centres, or groups of cells, which certainly exercise 

 a command over the heart and other organs, the 

 working of which is governed by the sympathetic 

 nervous system. It has already been shown in 

 dealing with the heart how, for example, the vagus 

 nerve has the power of slowing the heart's move- 

 ments. So also, the regular movements of breathing 

 may be regarded as controlled from the spinal cord, 

 and in its lower portion centres exist exercising a 

 certain amount of governance over the functions of 

 the bowel, bladder, and other organs. The spinal 

 cord is thus seen to be a very effective deputy of the 

 brain itself, and the principle of labour division in 

 the nervous system may find an illustration in the 

 share which the spinal cord assumes in the control 

 of the bodily actions, leaving the brain, so to speak, 

 free for the discharge of more important duties. 



THE EVOLUTION OF THE BRAIN. It is possible to 

 trace from the fish upwards to man, an identity of 

 type or plan in the build of the brain. In fishes, 

 frogs, and reptiles the brain is represented by a series 

 of masses of nerve cells placed, more or less 

 distinctly, in line. For convenience sake, we may 

 enumerate these masses as consisting of the olfactory 



