Nervous System and Sense Organs 207 



the autonomic system. The nerve cell bodies are located in the ganglia. 

 Fibers from these ganglia supply all parts of the body, and are un- 

 myelinated. 



The second portion of the autonomic nervous system, the para- 

 sympathetic division, has its origin in the mesencephalon, the myelen- 

 cephalon, and the sacral portion of the spinal cord. Like the other 

 division, the nerve processes of the parasympathetic division synapse 

 in ganglia. In general, however, these ganglia are much closer to the 

 organ which is innervated. The tenth cranial nerve, the vagus, is 

 one of the chief nerves of this division. Each of the visceral organs 

 has fibers innervating it from each of these two major divisions. 



Physiologically these two divisions of the autonomic nervous system 

 are distinguished from one another. 



The endings of the nerves of the sympathetic division secrete a 

 substance known as sympathin which is similar in its action to the 

 adrenaline secreted by the medulla of the adrenal gland. The endings 

 of the nerves of the parasympathetic division secrete another material, 

 acetylcholine. The effect of these materials on the various organs ap- 

 pears to be dependent to a great extent upon the organs themselves. 

 Thus sympathin accelerates the heart but inhibits movements of the 

 digestive system; acetylcholine decreases the heart rate but augments 

 the movements of the digestive tract. 



It would appear that these substances could spread from one 

 organ to another as they are liberated from the nerve endings. With 

 acetylcholine, at least, this is prevented by the presence of a special 

 enzyme in the blood, acetylcholine esterase, which immediately destroys 

 the acetylcholine. Sympathin is not destroyed so rapidly, but after 

 a few seconds it is oxidized and becomes inactive. 



THE PHYSIOLOGY OF NERVE CONDUCTION 



During the discussion of the structure of the nervous system, it 

 was necessary to refer to "nerve impulses," "reflex arcs," etc.; now 

 it is worth while to examine the exact nature of the nerve impulse and 

 its relationships to the structure of the nervous system. 



The Nerve Impulse.— There is still considerable question as to 

 the exact nature of the nerve impulse. At present, however, the most 

 generally accepted theory is that known as the membrane theory (Fig. 

 65). According to this, the surface of the individual nerve fiber is 

 a semipermeable membrane which is polarized when the nerve is at 



