252 COMPARATIVE PHYSIOLOGY. 



value ; but it is clear to those wlio have devoted attention to 

 comparative physiology that the more this subject is extended 

 the better prepared we shall be for taking a broad and soimd 

 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: 

 (6) intra-cardiao 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 moUusk (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 

 in action, neighboring heart-cells to contraction. The ability 

 that one protoplasmic cell-mass has to initiate in others, under 

 certain circumstances, like conditions with its own, is worthy 

 of more serious consideration in health and disease than it has 

 yet received. 



5. The influence of the cardiac nerves becomes more pro- 

 nounced as we ascend the animal scale. Their share in the 

 heart's beat will be considered later. 



6. Apparently in all hearts there is a functional connection 

 leading to a regular sequence of beat in the different parts, in 

 which the sinus or its representatives (the terminations of great 

 veins in the heart) always takes the initiative. One part having 



