180 ANIMAL PHYSIOLOGY. 



corpuscles throughout the rest of the nervous system. Its 

 peculiarity lies in a sympathy between it and the niind, 

 whereby centripetal action which has reached it affects the 

 mind, and the mind in turn excites nervous action in it, 

 which may extend centrifugally to the voluntary muscles. 



The sum of the function of the nervous system is, that it 

 conveys and distributes nervous impression. The waves of 

 impressed condition, or nervous impulses, may originate either 

 from external irritation applied to terminal organs, or from 

 an unknown description of irritation exerted by the mind ; 

 and in some instances they influence the mind, while in 

 others they act on muscles and glands. Further, it must be 

 admitted that no mental action can take place without 

 nervous action; but it is distinctly to be understood that 

 nervous impression, or the active state of the nervous system, 

 is not the same thing as mental action, but is a definite 

 physical change, which, there is no reason to doubt, is always 

 of one description. 



135. Living nerve resembles living muscle in being in a 

 peculiar state of electric tension when at rest, and in chang- 

 ing its electric condition when in action. Detached portions 

 of nerve examined by means of the galvanometer give results 

 similar to those obtained from blocks of muscle (p. 57); but 

 the currents being much weaker, a finer instrument is required 

 for their detection. Nerve has further the remarkable pro- 

 perty, that if a portion be placed within the circuit of a 

 galvanic current, a current in the same direction is produced 

 in the whole length of the nerve : the nerve is then said to 

 be in a state of electrotonus. A galvanic current applied to 

 a trunk of nerve supplying a muscle does not maintain the 

 muscle in contraction ; but there is a contraction every time 

 that the circuit is completed, and every time that it is broken, 

 so that the muscle can only be kept contracf ed by a con- 

 stantly interrupted current. In electrotonus the nerve still 

 performs its functions, but its degree of irritability is altered 

 in different parts of its course. These facts show that nervous 

 influence is not a current of electricity. By applying the 

 stimulus to different points on the nerve trunk, and measur- 

 ing, with an instrument called a myographion, the difference 

 in time between application of the stimulus and contraction 



