262 ANIMAL PHYSIOLOGY. 
differences for each group of animals; thus, these ganglion-' 
cells are most abundant, so far as the mammals as yet inves- 
tigated are concerned, in the ventricles of the pig, and least so 
in those of the dog. In the cat they are also scanty. Ganglion- 
cells occur in the auricles, and are especially abundant near the 
terminations of the great veins. 
It has long been known that the heart of a frog removed 
from the body will pulsate for hours, especially if fed with 
serum, blood, or similar fluids; and that it may be divided in 
almost any conceivable way, even when teased up into minute 
particles, and still continue to beat. The apex, however, when 
separated does not beat. Yet even this quiescent apex may be 
set pulsating if tied upon the end of a tube, through which it 
may be fed under pressure. 
We may here point out that the whole heart or a part of it 
may be made to describe its action by the graphic method in 
various ways, the principles underlying which are either that 
the heart pulls upon a recording lever (lifts it) acts against the 
fluid of a manometer; or, inclosed in a vessel containing oil or 
similar fluid, moves a piston in a cylinder. 
It has also long been known that a ligature drawn around 
the sinus venosus (in the frog) at its junction with the auricles 
stopped the heart for a certain period, and this experiment (of 
Stannius) was thought to demonstrate that the heart was ar- 
rested because the nervous impulses proceeding to the ganglion- ' 
cells along the cardiac nerves or ganglia of this region were 
cut off by the ligature; in other words, the heart ceased to beat 
because the outside machinery on which the action of the inner 
depended was suddenly disconnected. Other explanations have 
been offered of this fact. 
Within the last few years great light has been thrown upon 
the whole subject of cardiac physiology in consequence of in- 
vestigators having studied the hearts of various cold-blooded 
animals and of several invertebrates. The hearts of the Che- 
lonians (tortoises, turtles) have received special attention, and 
their investigation has been fruitful of results, to the general 
outcome of which, as well-as those accruing from recent com- 
parative studies as a whole, we can alone refer. 
Very briefly, the following are some of the main facts: 
1. In all cold-blooded animals the order in which the sub- 
divisions of the heart cease to pulsate when kept under the 
same conditions is invariable, viz., ventricle, auricles, sinus. 
2. The sinus and auricles, when separated by section, liga~ 
