NERVES OF THE HEART. CARDIAC MUSCLE. 259 



a set of several beats when directly stimulated, as by pricking 



or by induction shocks. The loss of irritability as the 



heart dies also usually appears in the same order: when the 



ventricle and auricle have both ceased to beat, it is frequently 



possible to excite the auricle by a direct stimulation which is 



powerless when applied to the ventricle; and when the whole 



heart has ceased to pulsate the venous sinus will sometimes re- 



spond to direct stimulation when auricle and ventricle will not. 



Still further, if the heart be carefully divided at the level aa, 



Fig. 99, so as to separate the sinus from the 



rest, the usual result is that the sinus goes on 



beating, but the rest of the heart lies for a 



time at rest: soon it begins to beat quite 



rhythmically, but at a slower rate than the 



separated sinus. If the cross-section be made 



at the level gg so as to separate the sinus and 



auricle from the rest, they go on beating, but 



the ventricle and bulb usually lie quiescent 



for a considerable time, and then commence. 



On account of the anatomical relations of the 



parts (Z, Fig. 99) it is not possible to com- 



pletely separate the ventricle from the sinus O f a pert uskm 



without doing injury to the former; but if separated apex t of 



the lower third of the ventricle (which con- f?o g 'J e heart e fo? f the 



tains no nerve-cells) be cut off from the rest 



of the heart along the line oo, this separated jfj 



portion never begins to beat spontaneously, uia 



though the remainder of the heart continues its pulsations. 



So far the case for the view that the nerve-cells take the in- 



itiative in the changes which result in a normal beat, and 



that cardiac muscle is not automatic, is a strong one; but 



other facts show that it cannot be accepted without modifi- 



cation. 



Although the separated apex of the ventricle of the frog, 

 left to itself, does not beat, yet it can be made to beat without 

 the application to it of anything that we are justified in call- 

 ing a stimulus: it does under certain conditions exhibit auto- 

 maticity. If it be tied on the end of a tube divided by a 

 partition (Fig. 100), and some blood or blood-serum be circu- 

 lated through it, in from a and out by b, under a slight press- 

 ure, this bit of ventricle, devoid of nerve-cells, after a time 

 begins to beat rhythmically. It has been suggested that in 



$ Jan- 



