THE CIRCULATION OF THE BLOOD. 



233 



is shown by the action of the heart within the body; its systole is fol- 

 lowed by its diastole in regular sequence throughout the life of the 

 individual. The force and frequency of the systole may vary from time 

 to time as occasion requires, but there is no interruption to the action 

 of the normal heart, or any interference with its rhythmical contrac- 

 tions. Further, we find that in an animal rapidly bled to death, the heart 

 continues to beat for a time, varying in duration with the kind of ani^ 

 mal experimentally dealt with, and in the entire absence of blood within 

 the heart-chambers; and still further, if the heart of an animal be re- 

 moved from the body, it still, for a varying time, continues its alternate 

 systolic and diastolic movements. Thus we see that the power oi 

 rhythmic contraction depends neither upon connection with the centra] 



'C.S.d 



Fig. 191. ^FI& 191 A. 



Fig. 191. The heart of a frog (Rana esculenta) from th# fr^nT^V, Ventricle; Ad, right ouriJ 

 As, left auricle; B, bulbus arterio-us, dividing into right and left aortae. (Ecker.) 



Fig. lai A. The heart of a frog (Rana esculenta) from the back. s. v., Sinus venosus openeo. ; 

 c. s. s., left vena cava superior; c. s. d., right vena cava superior; c. /., vena cava irforior ; v. , v 

 vena puhnouales; A.d., right auricle; A.s., left auricle; A.p., opening of communication Jyefween thf, 

 right auricle and the sinus venosus, X 2^-3. (Ecker.) 



nervous system, nor yet upon the stimulation produced by the presence 

 of blood within its chambers it is automatic. The cause of this 

 rliyjthmic power has been the subject of much discussion and experi- T 

 mental observation. Up to a comparatively short time ago, the remark- 

 able property of the heart to continue its rhythmical contractions after T^ 

 removal from the body was believed to be connected in some way or (/ 

 other with the presence of collections of nerve cells, or ganglia in sev- 

 eral parts of its tissue. Although this idea, as we shall presently see, has 

 now been very generally given up, it may be as well to describe shortly 

 these ganglia in this place; they have been studied more particularly 



