196 



THE CIRCULATION OF THE BLOOD 



will take place, as we have mentioned, without the presence of blood or 

 other fluid within its chambers. Not only is this the case, but the auricles 

 and ventricle may be cut off from the sinus, and both parts continue to pul- 

 sate; and, further, the auricles may be divided from the ventricle with the 

 same result. If the heart be divided lengthwise, its parts will continue to 

 pulsate rhythmically. The ventricle remains comparatively quiet, contrac- 

 tions occurring at longer intervals, if at all. However, the ventricle remains 

 irritable so long as bathed in blood, and will contract upon receiving a slight 

 stimulus; in fact, a single stimulus will often call forth a series of contractions 

 of the ventricle. The frog's ventricle, when its muscular and nervous con- 



FIG. 175. Isolated Nerve Cells from the Frog's Heart. 7, Usual form; II, twin cell; C, 

 capsule; N, nucleus; P, process. (From Ecker.) 



nections with the auricle are physiologically severed, as by crushing, will 

 remain quiet when fed by its own blood, but contracts rhythmically when 

 fed with physiological salt solution. 



It will be thus seen that the rhythmical movements appear to be more 

 marked in the parts supplied by the ganglia, that ventricular pieces con- 

 tract when still connected with the auricles, and that rhythmic contractions 

 of the ventricles do not readily occur in the ordinary condition when irri- 

 gated with blood. These are regarded as facts peculiarly in favor of the 

 neurogenic theory. 



The Myogenic Theory. In the myogenic theory the heart's rhythmical 

 contractions are explained as due to the inherent property of the cardiac 

 muscle itself. Most convincing facts in support of this theory have been 

 arrived at by a study of cardiac muscle, as such, and by studies on the whole 

 heart, particularly by Gaskell's method of blocking. The term blocking 

 is explained as follows: It will be remembered that under normal con- 

 ditions the wave of the contractions in the heart starts at the sinus and 

 travels down over the auricles to the ventricles, the irritability of the muscle 

 and its power of rhythmic contractions being greatest in the sinus, less in 

 the auricles, and least in the ventricles. By an arrangement of ligatures 

 or by a system of clamping one part of the heart may be more or less isolated 



