THE CIRCULATION OF THE BLOOD AND LYMPH 133 



been sometimes, though erroneously, brought forward as evidence 

 of myogenic automatism. Thus the supposedly ganglion-free 

 apex of the frog's heart, lifeless as it seems when left to itself, 

 can be caused to execute a long and faultless series of pulsations 

 when its cavity is distended with denbrinated blood or serum, 

 or certain artificial nutritive fluids, or even physiological salt 

 solution. The passage of a constant current through the 

 preparation may also start a regular rhythm. But apart from 

 the question whether nervous elements would not be subjected 

 to the constant stimulus impartially with the muscular elements 

 (and nerve-fibres, at any rate, are acknowledged to be present), 

 the beat here produced ought not to be considered as an auto- 

 matic beat, but as a contraction evoked by an external stimulus. 

 Such experiments, in fact, throw no light upon the automatism 

 of the heart, but prove clearly its rhythmicity i.e., its power of 

 responding to a continuous stimulus by regularly recurring 

 contractions. While we are hardly at present in a position to 

 discriminate sharply between the influence of constant stimula- 

 tion upon the nervous and upon the muscular elements of the 

 heart, and certainly not in a position to deny to the nervous 

 elements the power of responding to such stimulation by rhyth- 

 mical discharges, it can hardly be doubted that the cardiac muscle 

 itself possesses rhythmical power. This is a property which also 

 belongs to the smooth muscle of such tubes as the ureter, whose 

 rhythmical contraction is affected by distension much as that of 

 the heart is, and in a smaller degree even to ordinary skeletal 

 muscle, which can contract with a kind of rhythm under the 

 stimulus of a certain tension and in certain saline solutions. 

 But just as the primitive automatism of the cardiac muscle may 

 have become subordinated in the course of development to the 

 automatism of the nervous elements, so the primitive rhythmical 

 power of the muscle may under ordinary conditions remain in 

 abeyance and yet be capable of asserting itself in favourable 

 circumstances, and when the normal rhythmical impulses from 

 the nervous apparatus are withdrawn. In any case, in the 

 normally beating heart the opportunity for the exercise of the 

 rhythmical power of the muscle does not arise, at least in the 

 case of the lower portions of the heart. For the impulses which 

 (in the frog's heart), descending from the sinus, liberate the 

 contraction ot the auricles, and the impulses which, descending 

 from the auricles, liberate the contraction of the ventricle appear 

 to be discrete, and not continuous ; in other words, the lower 

 portions of the heart do not receive from the upper portions a 

 continuous stream of stimuli to which they respond by rhyth- 

 mical contractions, but a series of rhythmically repeated impulses, 

 each of which evokes a single contraction. One of the best 



