AND ITS MODE OF ACTION. 



891 



Fig. 133. 



glandular organs, owing to the stoppage of the circulation, disappears 

 also very rapidly, or at least cannot readily be demonstrated. The 

 contractility of the muscles, however, lasts, as we have seen, for a 

 considerable time after death, and may accordingly be employed 

 with great readiness as a test of nervous irritability. The manner 

 of its employment is as follows : 



The leg of a frog is separated from the body and stripped of its 

 integument; the sciatic nerve having been previously dissected 

 out and cut off at its point of emergence from the 

 spinal canal, so that a considerable portion of it 

 remains in connection with the separated limb. 

 (Fig. 133.) If the two poles of a galvanic appa- 

 ratus be now placed in contact with different 

 points (a b) of the exposed nerve, and a discharge 

 allowed to pass between them, at the moment 

 of discharge a sudden contraction takes place in 

 the muscles below. It will be seen that this ex- 

 periment is altogether different from the one re- 

 presented in Fig. 132. In that experiment the 

 galvanic discharge passes through the muscles 

 themselves, and acts upon them by direct stim- 

 ulus. Here, however, the discharge 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 pecu- 

 liar agency, causes in this way a muscular con- 

 traction. It is evident that in order to produce 

 this effect, two conditions are equally essential : 1st. 

 The irritability of the muscles ; and 2d. The irri- 

 tability of the nerve. So long, therefore, as the 

 muscles are in a healthy condition, their contraction, under the 

 influence of a stimulus applied to the nerve, demonstrates the irri- 

 tability of the latter, and may be used as a convenient measure of 

 its intensity. 



The irritability of the nerve continues after death. The knowledge 

 of this fact follows from what has just been said with regard to ex- 

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

 bility of the nerve, like that of the muscle, depends directly upon 

 its anatomical structure and constitution ; and so long as these re- 

 main unimpaired, the nerve will retain its vital properties, though 

 respiration and circulation may have ceased. For the same reason, 



FROG'S LEO. with 

 sciatic nerve (X) at- 

 tached. a b. Pole* of 

 galvanic battery, ap- 

 plied to nerve. 



