THE CIRCULATION OP THE BLOOD. 251 



I 



careful search has revealed no nerve-cells, yet their hearts con- 

 tinue to beat when their nerves are severed, on section of parts 

 of the organ, etc. 



6. A strip of the muscle from the ventricle of the tortoise, 

 when placed in a moist chamber and a current of electricity 

 passed through it for .some hours, will commence to pulsate and 

 continue to do so after the current has been withdrawn ; and 

 this holds when the strip is wholly free from nerve-cells. 



From the above facts certain inferences have been drawn : 

 1. It has been concluded that the sinus is the originator and 

 director of the movements of the rest of the heart. 2. That this 

 is owing to the ganglia in its walls. While all recognize the 

 importance of the sinus, some physiologists hold to the gangli- 

 onic influence as essential to the heart-beat still ; while others, 

 influenced by the facts mentioned above, are disposed to regard 

 them as of very doubtful importance — at all events, as origina- 

 tors of the movements of the heart. 



The tendency now seems to be to attach undue importance 

 to the spontaneous contractility of the heart-muscle ; for it by 

 no means follows logically that, because a muscle treated by 

 electricity, when cut off from the usual nerve influence that we 

 believe is being constantly exerted on the heart like other or- 

 gans, will contract and continue to do so in the absence of the 

 stimulus, it does so normally ; or, because some hearts beat in 

 the absence of nerve-cells, that therefore nerve-cells are of no 

 account in any case. Such views, when pressed to the extreme, 

 lead to as narrow conceptions as those they are intended to re- 

 place. 



Taking into account the facts mentioned and others we have 

 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-blooded animals is greater than of the mammalian 

 heart ; while, so far as exact or experimental knowledge is con- 

 cerned, the human heart is the least understood of all, though 

 there is evidence of a pathological and clinical kind and subject- 

 ive experience on which to base conclusions possessing a certain 



