THE ANIMAL A MACHINE 193 



1. Various modes of stimulation. A nerve can be stimulated electrically, 

 or by irritant chemical agents such as acids, or simply by striking, tearing 

 and stretching. 



The traveling charge on the iron wire can be initiated not only by an 

 electrical current, but by acids or other substances that touch the wire. 

 Tapping or bending the wire will do it. This finding is not surprising 

 since the charge of a battery in an automobile drops after violent shaking 

 by riding over rough roads, as many a driver knows from experience. 



2. I m parlance of the direct ion of the flow of the current. In order to charge 

 a battery the current must be sent through it in the right direction. Many a 

 driver has found out that his battery was dead after it had been connected 

 with the generator with the poles reversed. 



If the nerve impulse is comparable to a traveling electric charge, one 

 anticipates that the direction of the flow is of prime importance. This is 

 found to be the case: stated in scientific terms, stimulation is a "polar 

 phenomenon." 



3. The "refractory period." For a short time after an impulse has passed 

 along a nerve-fiber, the fiber does not conduct. It has become "refrac- 

 tory." In exactly the same manner the iron wire fails to conduct after 

 one charge has passed over it; a second charge immediately applied will 

 not travel on. It certainly is noteworthy that even this special feature of 

 nervous excitability finds its analogy. In the wire as well as in the nerve 

 this "refractory period" keeps the waves traveling in one direction. The 

 wave cannot run back over that portion of the nerve, or wire, over which it 

 has just traveled since this portion does not conduct. 



4. Rhythmic nerve impulses and waves. In most living animals we 

 observe periodic and automatic movements, of which the beating heart is 

 the best known example. Phigines include mechanical devices such as the 

 distributor, which ignites the cylinders at regularly repeated intervals, 

 timed synchronously with the movements of the valves. In a beating heart, 

 there is also harmonious timing of all its different muscles, accomplished 

 without any rotating device or anything remotely resembling a timing 

 shaft. The living organism, has means that are far simpler and yet as effec- 

 tive as complicated modern machinery. In the human heart, which beats 

 75 to 80 times a minute, each beat is initiated by a nerve impulse originating 

 in the sinus node in the upper portion of the heart. From this point a 

 bundle of fibers passes downward; the impulse thus spreads and causes a 

 contraction of the various parts of the heart, one after the other. The 

 most mysterious part of the action is the origin of the impulse in the sinus 

 node. 



Yet there is nothing peculiarly "vital" in such a rhythm. Iron wires 

 immersed in nitric acid likewise show it. The best method of observing 



