258 A Manual of Veterinary Physiology. 



The explanation of electrotonus in nerves is that it is a 

 vital phenomenon, viz., the irritability of the nerve is 

 increased when its molecules pass from their ordinary con- 

 dition to one of greater mobility (kathelectrotonus), or it 

 is diminished when its molecules pass from their ordinary 

 condition to one of less mobility (anelectrotonus) (Cyon). 

 Hermann considers that it is a purely physico-chemical 

 phenomenon, due to the electric current generating acids at 

 the positive pole, and alkalis at the negative : the effect of 

 the acid being to lower the excitability of the nerve, and 

 of the alkali to increase it. 



The practical application of this law is that the excita- 

 bility of a part, pain, cramp, etc., may be removed by 

 passing a current up the nerve, viz., by placing the positive 

 pole nearest the muscle, and producing anelectrotonus ; or 

 by reversing the process and throwing the current down 

 the nerve, so that the negative pole is nearest the muscle, 

 the irritability of the part is increased. 



The Irritability of Nerves is increased by electric, 

 thermal, chemical, and other stimuli : it is diminished by 

 cold, compression, or injur}-: it is exhausted by shock or 

 continual excitement. Nerves are not capable of originating 

 in themselves any impulse ; the latter is produced by the 

 impressions made on the nerve terminations. While im- 

 pulses are being conducted along a nerve, neighbouring 

 nerves, though in contact, are not affected by it. This is 

 probably explained by the fact that each nerve fibre is 

 isolated from its fellow by a sheath, which acts the part of 

 a non-conductor. A nerve cannot be motor one moment 

 and sensory the next. These functions are entirely dis- 

 tinct, and are dependent upon the nerve terminations. 



It is necessary for the transmission of an impression that 

 the nerve should be intact from its origin to its destination : 

 division of a sensory nerve means loss of sensation to all 

 parts supplied by it ; if, the main trunk be divided, entire 

 sensation is lost; if only a branch of the trunk, loss of 

 sensation follows to the part supplied. We sec this per- 

 fectly demonstrated in the ordinary operation of neurectomy. 



