THE CIRCULATION OF THE BLOOD. 



269 



be less under nervous control, but it certainly is least dependent 

 on the nervous system, and has the greatest automaticity. 



The law of rhythm in organic nature finds some of its most 

 €!vident exemplifications ia the circulation. Most of the 

 rhythms are com- 

 pound, one being 

 blended with or su- 

 perimposed on an- 

 other. Even the ap- 

 parent irregularities 

 of the normal heart 

 are rhythmical, such 

 as the very marked 

 slowing and ,other 

 changes accompany- 

 ing expiration, espe- 

 cially in some ani- 

 mals. 



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 es- 

 timating any one of 

 the factors of the cir- 

 culation. We know 

 a good deal at present of cardiac physiology, 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. e., we 

 are not in a position to state the laws that apply to all forms of 

 heart. 



Summary of the Physiology of the Cironlation.— In the 

 mammal the circulatory apparatus forms a closed system con- 

 sisting 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 



Pia. 823.— yeins of the foot of the horse (after Chau- 

 veau). 



