300 



ADVANCED PHYSIOLOGY 



from which messages go over the vagus nerves to check the 

 beating of the heart; see page 145. Although the above are 



the most important, there are other 

 centers located in the medulla; but, 

 in general we may say that the 

 medulla is the seat of all involuntary 

 activities. 



THE CONNECTIONS BETWEEN THE 

 BRAIN AND THE BODY 



The brain, shut up as it is within 

 the bony walled cranium, may be 

 compared to a telegraph operator in 

 his small office. It is quite remote 

 from many important organs of the 

 body, but by means of innumerable 

 nerve fibres, corresponding to tele- 

 graph wires, it is connected with 

 them all, as the telegraph operator 

 may be in communication with the 

 rest of the world. The next step 

 in our discussion, then, is to study 

 the spinal cord, which is the main 

 cable, as it were, of nerve fibres. 



THE SPINAL CORD 



Since the term spine is commonly 

 applied to the backbone, the large 

 nerve which passes down through 

 it is naturally called the spinal cord. 

 Notwithstanding the innumerable 

 twists and bends which the body 

 makes, the cord is perfectly protected from strain and injury. 

 Figure 123 shows that each vertebra is essentially an irregular 

 ring of bone encircling an opening; when a number of vertebrae 



FIG. 153. THE SPINAL 

 CORD 



With the spinal nerves at- 

 tached. Upon one side is 

 shown the sympathetic 

 system. (Thompson) 



