AND ITS MODE OF ACTION. 



419 



apparatus (Fig. 143, a, 6) applied to the surface of Fig. 143. 



the denuded muscles. A contraction takes place 



each time the circuit is completed, when the elec- 



tric discharge passes through the limb. In this case, 



the stimulus is applied directly to the muscles, and 



shows that their irritability, or power of contrac- 



tion under the influence of an exciting cause, does 



not depend upon their remaining in connection 



with the nerves or nervous centres. A single mus- 



cular fibre, in fact, separated from all neighboring 



parts, may sometimes be seen to contract under the 



microscope for a certain time after its removal from 



the muscular tissue. The muscles will also respond 



by contraction to various other kinds of mechanical 



or chemical stimulus, such as pinching, pricking, 



cauterizing, the contact of hot or cold bodies, or FROG'S LEG, with 



the application of various acid, alkaline, or saline vanic battery applied 



solutions. The most efficient and manageable stimu- to the muscles at 



lus, however, is the electric discharge. 



Mode of exhibiting Nervous Irritability. In order to exhibit the 

 irritability of the motor nerve fibres, a frog's leg is prepared, as in the 

 preceding experiment, except that the sciatic nerve is cut off at its 

 point of emergence from the spinal canal, and dissected from the adja- 

 cent tissues, so that a considerable portion of it is left exposed, but 

 retaining its connection with the separated limb 

 (Fig. 144). If the two poles of a galvanic battery 

 be now placed in contact with different points (a, 6) 

 of the exposed nerve, and a current allowed to pass 

 between them, at the moment of its passage a con- 

 traction takes place in the muscles below. It will 

 be seen that this experiment is altogether different 

 from the one represented in Fig. 143. In that case 

 the electric discharge passes through the muscles 

 themselves, and acts upon them by direct stimulus. 

 Here the current passes only from a to b through 

 the tissues of the nerve, and acts directly upon the 

 nerve alone; while the nerve, acting upon the 

 muscles by its own special agency, causes in this 

 way a muscular contraction. So long, therefore, 

 as the muscles are in a healthy condition, their con- 

 traction, under the influence of a stimulus applied 

 to the nerve, demonstrates the irritability of the 

 latter, and may be used as a convenient measure of 

 its intensity. 



The irritability of a motor nerve continues after 

 death. The knowledge of this fact follows from the "cittto 

 what has been said with regard to experimenting attached 

 upon the frog's leg, prepared as above. The irrita- 



Fig. 144. 



b, Poles 



