SEC. 5. THE CIRCUMSTANCES WHICH DETERMINE 

 THE DEGREE OF IRRITABILITY OF MUSCLES AND 

 NERVES. 



83. A muscle-nerve preparation, at the time that it is re- 

 moved from the body, possesses a certain degree of irritability, it 

 responds by a contraction of a certain amount to a stimulus of a 

 certain strength, applied to the nerve or to the muscle. After a 

 while, the exact period depending on a variety of circumstances, 

 the same stimulus produces a smaller contraction, i.e. the irritability 

 of the preparation has diminished. In other words, the muscle 

 or nerve or both have become partially ' exhausted ' ; and the 

 exhaustion subsequently increases, the same stimulus producing 

 smaller contractions, until at last all irritability is lost, no stimulus 

 however strong producing any contraction whether applied to the 

 nerve or directly to the muscle ; and eventually the muscle, as we 

 have seen, becomes rigid. The progress of this exhaustion is more 

 rapid in the nerves than in the muscles ; for some time after the 

 nerve-trunk has ceased to respond to even the strongest stimulus, 

 contractions may be obtained by applying the stimulus directly to 

 the muscle. It is much more rapid in the warm-blooded than in 

 the cold-blooded animals. The muscles and nerves of the former 

 lose their irritability, when removed from the body, after a period 

 varying according to circumstances from a few minutes to two or 

 three hours ; those of cold-blooded animals (or at least of an 

 amphibian or a reptile) may under favourable conditions remain 

 irritable for two, three, or even more days. The duration of 

 irritability in warm-blooded animals may however be considerably 

 prolonged by reducing the temperature of the body before death. 



If with some thin body a sharp blow be struck across a muscle which 

 has entered into the later stages of exhaustion, a wheal lasting for 

 several seconds is developed. This wheal appears to be a contraction 

 wave limited to the part struck, and disappearing very slowly, without 

 extending to the neighbouring muscular substance. It has been called 



