264 ANIMAL PHYSIOLOGY. 
not space to enumerate, we submit the following as a safe view 
to entertain of the beat of the heart in the light of our present 
knowledge: 
Recent investigations show clearly that there are great dif- 
ferences in the hearts of animals of diverse groups, so that it 
is not possible to speak of “the heart” as though our remarks 
applied equally to this organ in all groups of animals. 
It must be admitted that our understanding of the hearts of 
the cold-blood animals is greater than of the mammalian heart; 
while, so far as exact or experimental knowledge is concerned, 
the human heart is the least understood of all, though there is 
evidence of a pathological and clinical kind and subjective 
experience on which to base conclusions possessing a certain 
value; but it is clear to those who have devoted attention to 
comparative physiology that the more this subject is extended 
the better prepared we shall be for taking a broad and sound 
view of the physiology of the human heart and man’s other 
organs. 
Whatever may be said of the invertebrates, among which 
greater simplicity of mechanism doubtless prevails, there can 
be no doubt that the execution of a cardiac cycle of the heart 
in all vertebrates, and especially in the higher, is a very com- 
plex process from the number of the factors involved, their in- 
teraction, and their normal variation with circumstances; and 
we must therefore be suspicious of any theory of excessive sim- 
plicity in this as well as other parts of physiology. 
We submit, then, the following as a safe provisional view of 
the causation of the heart-beat : 
1. The factors entering into the causation of the heart-beat 
of all vertebrates as yet examined are: (a) A tendency to spon- 
taneous contraction of the muscle-cells composing the organ; 
(b) intra-cardiac blood-pressure; (c) condition of nutrition as 
determined directly by the nervous supply of the organ and in- 
directly by the blood. 
2. The tendency to spontaneous contraction of muscle-cells 
is most marked in the oldest parts of the heart (e. g., sinus), 
ancestrally (phylogenetically) considered. 
3. Intra-cardiac pressure exercises an influence in determin- 
ing the origin of pulsation in probably all hearts, though like 
other factors its influence varies with the animal group. In 
the mollusk (and allied forms) and in the fish it seems to be the 
controlling factor. 
4, We must recognize the power one cell has to excite when 
