146 



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 rhythmical discharges, 

 it cannot be doubted that tht cardiac muscle itself possesses rhythmi- 

 cal power. One of the best proofs of this is derived from observa- 

 tions on the growth of tissue cultures containing fragments of heart 

 muscle. A group of heart muscle cells which had become entirely 

 separated from the rest of the culture continued to beat rhythmically 

 although with a different rhythm from that of the original piece. 

 No ganglion cells or nerve fibres could be seen in it. Rhythmical 

 contraction was observed even in a single isolated heart muscle cell 

 (Burrows). The power of rhythmical contraction 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 sub- 

 ordinated 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 with- 

 drawn. 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 of 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 con- 

 tinuous stream of stimuli to which they respond by rhythmical con- 

 tractions, but a series of rhythmically repeated impulses, each of 

 which evokes a single contraction. One of the best proofs of this 

 is that if the sinus is heated the ventricle beats much more rapidly in 

 unison with the rapidly beating sinus and auricles, while if the 

 ventricle itself is heated no change takes place in its rhythm. Now, 

 if the ventricle responds to a constant stimulus by rhythmical beats, 

 the condition of the ventricular tissue ought to affect the rate of its 

 beat. In the mammalian heart, too, an alteration in the tempera- 

 ture of a definite area of the wall of the right auricle lying between 

 the mouths of the venae cavae produces a change in the rate of the 

 whole heart, while no effect is caused by altering the temperature 

 of any other portion of the heart. It has already been stated that 

 the impulses from the nerve-cord which maintain the rhythm in the 

 Limulus heart are also discontinuous. 

 Conduction and Co-ordination. The question of the conduction 



