260 ANIMAL PHYSIOLOGY. 
heart will pulsate when removed, either entire or after being 
divided into sections. 
In another specimen it would be desirable to allow the 
heart, to be kept bathed in serum or physiological saline solu- 
tion, to beat as long as it will, and to note the various phases 
of irregularity, weakening, and cessation of action in its dif- 
ferent parts. 
It is also highly instructive to observe the effect of ligating 
off certain of the chambers from the rest of the organ. 
Any one who makes a few such observations will be pre- 
pared to comprehend readily any of the experiments on the 
hearts of the cold-blooded animals, and will be able, especially 
if he has followed out earlier recommendations as to the study 
of the heart of the mammal, to form a mental picture of what 
is transpiring within his own breast, which is one of the most 
desirable accomplishments—in fact, the best test of real knowl- 
edge. 
Whatever ground for differences of opinion there may be 
as to the extent to which the phenomena we have as yet been 
describing are mechanical in their nature, all are agreed that 
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 met by 
changes in the circulatory system. These changes may affect 
any part of the entire arrangement, and it rarely happens, as 
will appear, that one part is modified without a correspond- 
ing 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 pict- 
ure 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. 
