AND ITS MODE OF ACTION. 



389 



these properties to a certain extent for some time afterward. It is 

 only when the constitution of the tissues has become altered by 

 being deprived of blood, and by the consequent derangement of 

 the nutritive process, that their characteristic properties are finally 

 lost. Thus, in the muscles, irritability and contractility may be 

 easily shown to exist for a short time after death by applying to 

 the exposed muscular fibre the same kind of stimulus that we have 

 already found to affect it during life. It is easy to see, in the 

 muscles of the ox, after the animal has been killed, flayed, and 

 eviscerated, different bundles of muscular fibres contracting irregu- 

 larly for a long time, where they are exposed to the contact of the 

 air. Even in the human subject the same phenomenon may be 

 seen in cases of amputation ; the exposed muscles of the amputated 

 limb frequently twitching and quivering for many minutes after 

 their separation from the body. 



The duration of muscular irritability, after death, varies consider- 

 ably in different classes of animals. It disappears most rapidly 

 in those whose circulation and respiration are naturally the most 

 active ; while it continues for a longer time in those whose circula- 

 tion and respiration are sluggish. Thus in birds the muscular 

 irritability continues only a few minutes after the death of the 

 animal. In quadrupeds it lasts somewhat longer ; 

 while in reptiles it remains, under favorable cir- 

 cumstances, for many hours. The cause of this 

 difference is probably that, in birds and quadrupeds, 

 the tissues being very vascular, and the molecular 

 changes of nutrition going on with rapidity, the 

 constitution of the muscular fibre becomes so 

 rapidly altered after the circulation has ceased, 

 that its irritability soon disappears. In reptiles, 

 on the other hand, the tissues are less vascular 

 than in birds and quadrupeds, and all the nutritive 

 changes go on more slowly. Kespiration and cir- 

 culation can therefore be dispensed with for a longer 

 period, before the constitution of the tissues be- 

 comes so much altered as to destroy altogether 

 their vital properties. 



Owing to this peculiarity of the cold-blooded 

 animals, their tissues may be used with great ad- 

 vantage for purposes of experiment. If a frog's leg, for example, 

 be separated from the body of the animal (Fig. 132), the skin 



Fig. 132. 



FROG'S LEO, with 

 poles of galvanic bat- 

 tery applied to the 

 muscles at a, b. 



