250 , COMPARATIVE PHYSIOLOGY. 



It has also long been known that a ligature drawn around 

 the sinus venosus (in the frog) at its junction with the auricles 

 stopped the heart for a certain period, and this experiment (of 

 Stannius) was thought to demonstrate that the heart was ar- 

 rested because the nervous impulses proceeding to the ganglion- 

 cells along the cardiac nerves or ganglia of this region were 

 cut off by the ligature ; in other words, the heart ceased to beat 

 because' the outside machinery on which the action of the inner 

 depended was suddenly disconnected. Other explanations have 

 been offe:red of this fact. 



Within the last few years great light has been thrown upon 

 the whole subject of cardiac physiology in consequence of in- 

 vestigators having studied the hearts of various cold-blooded 

 animals and of several invertebrates. The hearts of the Ghe- 

 lonians (tortoises, turtles) have received special attention, and 

 their investigation has been fruitful of results, to the general 

 outcome of which, as well as those accruing from recent com- 

 parative studies as a whole, we can alone refer. Since in other 

 parts of the work the limits of space will not always allow us 

 to give the evidence on which conclusions rest, attention is 

 especially called to what here follows, as an example of the 

 methods of physiological research, and the nature of the reason- 

 ing employed. 



Very briefly the following are some of the main facts : 



1. In all cold-blooded animals the order in which the sub- 

 divisions of the heart ceases to pulsate when kept under the 

 same conditions is invariable, viz., ventricle, auricles, sinus. 



2. The sinus and auricles, when separated by section, liga- 

 ture, or otherwise, either together or singly, continue to beat, 

 whether amply provided with or surrounded by blood. 



3. The ventricle thus separated displays less tendency to 

 beat independent of some stimulus (as feeding under pressure), 

 though a very weak one usually suffices — i. e. , its tendency to 

 spontaneous rhythm is less marked than is the case with the 

 other parts of the heart. These remarks apply to the hearts 

 of Chelonians- — fishes, snakes, and some other cold-blooded 

 animals. 



4. In certain fishes (skate, ray, shark) the beat may be re- 

 versed by stimulation, as a prick of the ventricle. This is 

 accomplished with more difficulty in other cold-blooded ani- 

 mals, and still more so in the mammal. 



5. In certain invertebrates, notably the Poulpe (Octopus), a 



