214 



it has lost its natural heat. The heart of a viper, or 

 tortoise, beats strongly for the space of twenty or thirty 

 hours after the death of th~ animal. Wate', or air, 

 when introduced into the ventricle, are sufficient to re- 

 store to the heart the motion it has lost. 



The peristaltic motion of the intestines is likewise 

 owing to their irritability. But the following is what \ve 

 should not have guessed at. If they are plucked hastily 

 from the lower belly, and cut into pieces, all these pieces 

 will crawl like worms, and contract themselves on the 

 slightest touch. 



80 tiiat not only every muscle, but also every frag- 

 ment of a muscle, and even every muscular fibr^, con- 

 tract themselves more or less on being touched by any 

 body whatsoever, especially if that body be of a stimu- 

 lating nature ; and as the fibre contracts, so it likewise 

 recovers of itself, and this alternate exercise lasts for a 

 time proportionable to the degree or' irritability. 



It is evident, from ail the experiments, that the vital 

 parts are the most irritable. The heart is the most ir- 

 ritable of all, and next to that, the intestines and dia- 

 phragm. 



The nature of irritability is unknown: we only judge 

 of it by its effects. It probably resides in the elastic 

 fluid which is interspersed between the lamelles of the 

 fibre. The nerves are not irritable ; but if a nerve be 

 pricked, the muscle at which it terminates will contract 

 itself. The nerves may then give motion to the mus- 

 cles; but they do not communicate an irritability to 

 them which they are not possessed of themselves, they 

 only put it into action - y and thus they are the ministers 

 of the affections of the soul. 



Irritability then seems to be what constitutes the 

 vital power in the animal ; arid this property has not 

 been perceived in the vegetable. Is it not then the dis- 

 tinguishing character we seek for! 



