286 ANIMAL PHYSIOLOGY. 
the auricles it may be, continues to beat when the ventricles 
have ceased; and we have learned that so long as these parts 
are functionally active there is a greater probability that the 
quiescent regions may recommence. Activity begets activity, 
in cardiac muscle-cells at least. How are these facts to be 
explained apart from evolution ? 
The law of rhythm in organic nature finds some of its most 
evident exemplifications in the circulation. Most of the 
rhythms are compound, one being blended with or superim- 
posed on another. Even the apparent irregularities of the nor- 
mal heart are rhythmical, such as the very marked slowing 
and other changes accompanying expiration, especially in some 
animals. 
We trust we have made it evident that the greatest allow- 
ance must be made for the animal group, and some even for 
the individual, in estimating any one of the factors of the cir- 
culation. We know a good deal at present of cardiac physiol- 
ogy, but we do not know a physiology of “the heart” in the 
sense in which we understand that term to have been used till 
recently—i. ¢., we are not in a position to state the laws that 
apply to all forms of heart. 
Summary of the Physiology of the Circulation —In the mammal 
the circulatory apparatus forms a closed system consisting of a 
central pump or heart, arteries, capillaries, and veins. All the 
parts of the vascular system are elastic, but this property is 
most developed in the arteries. 
Since the tissue-lymph is prepared from the blood in the 
capillaries, it may be said that the whole circulatory system 
exists for these vessels, 
As a result of the action of an intermittent pump on elastic 
vessels against peripheral resistance, in consequence of which 
the arteries are always kept more than full (distended), the 
flow through the capillaries and veins is constant—a very great 
advantage, enabling the capillaries to accomplish their work of 
feeding the ever-hungry tissues. While physical forces play a 
very prominent part in the circulation of the blood, vital ones 
must not be ignored. They lie at the foundation of the whole, 
here as elsewhere, and must be taken into the account in every 
explanation. 
As a consequence of the anatomical, physical, and vital char- 
acters of the circulatory system, it follows that the velocity of 
the blood is greatest in the arteries, least in the capillaries, and 
intermediate in the veins. ’ 
