830 CENTRAL NERVOUS SYSTEM 



The Nerve Net 



In the trunk of the sea anemone nervous tissue assumes a much more 

 important role. Between the receptors of the epithelium and the muscles 

 there is interposed a layer of nerve cells arranged as a network. This 

 network can transmit disturbances set up in the receptors to quite dis- 

 tant muscles, with the result that a local stimulus may bring about a gen- 

 eralized contraction of the trunk. The primitive nervous system is thus 

 seen to consist of a network of nerve cells or nerve net. Its charac- 

 teristic consists in the fact that the nerve cells are joined together by 

 continuous fibers so that the structure is essentially a syncytium. No 

 cell membranes can be distinguished separating the fibers of the constit- 

 uent nerve cells. The result of this structure is that nerve impulses set 

 up in one region can spread at random to all parts of the nerve net. The 

 responses produced by such a mechanism are necessarily diffuse in char- 

 acter, since large groups of muscles will be brought into play at one time. 

 Since conduction is not limited to definite paths in the nerve net, local in- 

 jury will have little effect on the reactions of the organism because the 

 nerve impulses can pass around the injured region through other parts 

 of the nervous network. The rapidity with which impulses are conducted 

 by the nerve net is not great, being only 146 centimeters a second, or about 

 two hundred times less than the velocity of the nerve impulse in the motor 

 nerves of the frog. 



The nerve net is retained in many parts of the higher animals. Wher- 

 ever it occurs it is usually very closely associated with the muscles which 

 it innervates. The most important nerve net in man is found in the 

 intestine (myenteric plexus). In this structure an important modifica- 

 tion in behavior has developed. While the reactions of the intestine 

 maintain the sluggish, diffuse character seen in the coelenterates, con- 

 duction no longer proceeds as readily in all directions. Excitation pro- 

 duces a contraction of the muscles above the point stimulated and relax- 

 ation below it (the myenteric reflex page 501). Waves of contraction 

 travel along the intestine usually only in one direction, from pyloric end 

 downward. The nerve net has developed a definite polarity, a property 

 which is fundamental in the central nervous system of the higher animals. 



The Central Nervous System 



A typical central nervous system appears in simple form in the seg- 

 mented worms. The nerve cells of which it is composed do not form 

 a syncytium, as in the nerve net, but are separated from one another at 

 the points at which their fibers meet by a specialized structure called 

 a synapse. A definite membrane may be seen at this point separating 

 the protoplasm of one fiber from that of the other. The nerve cell con- 



