248 COMPARATIVE PHYSIOLOGY. 



such explanations are insufficient when applied to the facts 

 with which we have yet to deal. They, at all events, can be 

 regarded only as the result of vitality. 



When one reflects upon the vicissitudes through which an 

 animal must pass daily and hourly, necessitating either that 

 they be met by modified action of the organs of the body or 

 that the destruction of the organism ensue, it becomes clear that 

 the varying nutritive needs of each part must be answered by 

 changes in the circulatory system. These changes may afiPect 

 any part of the entire arrangement, and it rarely happens, as 

 will appear, that one part is modified without a corresponding 

 one, very frequently of a different kind, taking place in some 

 other. What these various correlated modifications are, and 

 how they are brought about, we shall now attempt to describe, 

 and it will greatly assist in the comprehension of the whole if 

 the student will endeavor to keep a clear mental picture of the 

 parts before his mind throughout, using the figures and verbal 

 descriptions only to assist in the construction of such a mental 

 image. We shall begin with the vital pump — the heart. 



THE BEAT OF THE HEART AND ITS MODIFICATIONS. 



As has been already noted, the cardiac muscle has features 

 peculiar to itself, and occupies histologically an intermediate 

 place between the plain and the striped muscle-cells, and that 

 the contraction of the heart is also intermediate in character, 

 and is best seen in those forms of the organ which are somewhat 

 tubular and beat slowly. But the contraction, though peristal- 

 tic, is more rapid than is usually the case in organs with the 

 smooth form of muscle-fiber. 



The heart behaves under a stimulus in a peculiar manner. 

 The effect of a single induction shock depends on the phase of 

 contraction in which the heart happens to be at the moment of 

 its application. Thus at the commencement of a systole there 

 is no visible effect, while beats of unusual character result at 

 other times. But tetanus can not be induced by any form or 

 method of stimulation. The latent period of cardiac muscle is 

 long. 



In a heart at rest a single stimulus (as the prick of a needle) 

 usually calls forth but one contraction. 



