ANATOMY AND PHYSIOLOGY OF NERVOUS SYSTEM. 



experiment, it follows that a motor nerve can conduct an excitation 

 in both directions. 



Swiftness of the Nerve-wave. Compared with the rapidity of 

 an electrical current, the nerve-current is immeasurably slow. In 

 the motor nerves of a frog Helmholtz made it about 88 feet per second. 

 In the horse Chauveau found it to be about 227 feet per second in the 

 motor nerves of the larynx and only 24 feet in the motor nerves of the 

 oesophagus. In sensory nerves the velocity of the nerve-wave is 

 variable, but may be put down as 150 feet per second. Cold dimin- 

 ishes the swiftness of the nerve-wave. If the intensity of the elec- 

 trical stimulus is increased the swiftness is increased. The part of 

 a nerve in a state of an electrotonus slows the rapidity of the nerve- 

 current, and this is more perceptible as the duration and intensity 

 of the polarizing current increases. I have found that stretching a 

 nerve lowers the rate of transmission of nerve-force. The method of 

 Helmholtz to measure the velocity of the nerve-wave is as follows : 

 He stimulated a motor nerve of a muscle and registered the time 

 of its contraction after excitation. After a while the same nerve was 

 stimulated at a point nearer its distribution with the muscle. Its 

 time was also registered. The second time was found to be shorter 

 than the first, so that the difference between it and the preceding must 

 represent the time required between the two excitation points for the 

 transmission of the nerve-wave. The distance between the two stimu- 

 lated areas being known, one can very readily calculate the swiftness 

 of the nervous action. 



Excitability and Conductivity. Excitability of a nerve is its 

 ability to react to the irritations received by it, not only at one spot, 

 but through its whole length. Conductivity is the property of trans- 

 mitting through its whole length, up to its terminal extremity, a 

 nerve-wave which has been called out by an irritant. When a part of 

 a trunk of a sciatic nerve of a frog is submitted to the action of carbon 

 dioxide and you stimulate that part, no contraction ensues. But when 

 you stimulate the nerve above this point a tetanus ensues. Here the 

 nerve-wave must travel through the part affected by the carbon 

 dioxide. Hence it is inferred that conductivity and irritability are 

 separate properties in a nerve. 



Excitants of the Nerve. Nerve-excitants are all those forces 

 which modify its state. There are electrical, thermal, mechanical, 

 and chemical excitants. From -the fact that they may act upon a 

 nerve in any part of its course, they are frequently designated as 

 general stimuli. 



