146 THE CIRCULATION OF THE BLOOD AND LYMPH 



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 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 con- 

 traction 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 rhythmical impulses from the nervous apparatus are withdrawn.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 a 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 continuous 

 stream of stimuli to which they respond by rhythmical contractions, 

 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 

 temperature of a definite area of the wall of the right auricle lying 

 between the mouths of the venae cavse produces a change in the rate 

 of the whole heart, while no effect is caused by altering the tempera- 

 ture 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 

 of the excitation over the heart and the co-ordination of its parts 

 is in the same position as the question of the automatism and 

 rhythmicity. In the horseshoe crab, as already remarked, the 

 mechanism appears to be a nervous one. In higher hearts, on the 

 other hand, facts have been discovered which favour each of the 

 rival hypotheses. In the frog's heart the probability that the con- 



