IRRITABILITY OF NERVE-FIBRES. 109 



drawn from the influence of the circulation of the blood, 

 soon lose their power of activity, the nerves and muscles 

 of frogs are preferable for these experiments. The 

 lower part of the thigh of a frog, with a long portion of 

 the sciatic nerve, which is very easily separable up to 

 the point where it emerges from the vertebral column, 

 is best suited for this purpose. In some cases it is 

 better to use only the calf-muscle with the sciatic 

 nerve ; the muscle must be fastened in the same way 

 as in the former experiments, and its contractions must 

 be made evident by use of a lever. 



If the muscle, thus fastened, is pinched at any point 

 in its course it pulsates. The same result follows if a 

 thread is passed round the nerve, and the latter is thus 

 constricted, or if a small piece is cut from the nerve 

 with a pair of scissors. These are mechanical irrit- 

 ants which act on the nerve. Pulsation will, however, 

 also be seen if the nerve is smeared with alkaline 

 matter, or acid — these are chemical irritants. A por- 

 tion of the nerve may be heated; that is, it may be 

 thermicallv irritated. In all these cases, the nerve at 

 the point irritated, immediately, or, at least very soon, 

 loses its capacity for receiving irritation. But if the 

 nerve is placed on two wires, by means of which an 

 electric current is passed through one point in the 

 nerve, it may, in this way, be repeatedly electrically 

 irritated without its irritability being immediately de- 

 stroyed. It therefore appears that, in this respect, a 

 nerve acts exactly as does a muscle. If a constant 

 electric current is applied, the result is usually a pul- 

 sation on the closing and the opening of the current, 

 but sometimes a lasting contraction ensues while the 

 current flows through the portion of the nerve. If 



