48 Physiology. 



The brain starts the nerve currents, and they run along 

 the nerve as far as the compressed part ; here they stop. 

 On account of external pressure the nerve has temporarily 

 lost its power of conducting nerve currents. They cannot 

 reach the muscles of the leg below. Hence the muscles 

 do not shorten, and we do not rise, no matter how strongly 

 we will to do so. 



Dependence of Nerves and Muscles. But what beside 

 the nerve has been compressed ? What process in the 

 limb has been interfered with by the pressure due to the 

 position in which one has been sitting or lying ? What is 

 the temperature of the benumbed limb ? 



On what are the nerves and muscles so dependent for 

 keeping up their activity ? 



Summary. i . Motions are voluntary or involuntary, but all are 

 under control of the nervous system. 



2. The cerebro-spinal nervous system consists of the brain, the 

 spinal cord, and the spinal nerves. 



3. Each spinal nerve has two roots: the dorsal, which is afferent 

 and sensory ; the ventral, which is efferent and motor. 



4. A ganglion is a nerve center largely composed of nerve cells. 



5. Nerves are made up of nerve fibers. 



6. A nerve fiber consists of the central core (or axis cylinder), 

 which conducts the nerve impulse, the medullary sheath, and, outside, 

 the nerve-fiber sheath. 



7. The spinal cord has in its outer part white nerve fibers; in its 

 center, gray nerve cells. 



8. These cells are branched, and at least one branch becomes the 

 axis cylinder of a nerve fiber. 



9. The gray matter of the cord is the center of the reflex action. 



10. The nerve fibers from each half of the brain connect with the 

 opposite half of the body. 



11. The nervous system is comparable to a telegraph system. 

 Questions. i. Name as many involuntary motions as you know. 



2. What other cases of reflex action do you know ? 



3. Why is a man partially paralyzed when he has broken his back ? 



