188 THE CONTRACTION OF CARDIAC MUSCLE. 



inherent rhythmical power ; and, in the second place, Pratt, 1 working 

 with Porter, has shown that the isolated ventricle of the cat can be 

 tied on to a perfusion cannula and fed with blood at the temperature of 

 the body through the foramina Thebcsii, and then, just as in the case of 

 the frog's ventricle, it will beat regularly and well for many hours. 

 The strip from the mammalian ventricle, even when taken from the 

 apex of the ventricle, will beat regularly at its own rate of rhythm, 

 apart from the beating of the rest of the heart, provided that care be 

 taken to keep intact the circulation of blood through the coronary 

 vessels supplying the strip. 



Owing to this inherent rhythmical power of the mammalian 

 ventricle, when supplied with blood through the coronary system, it 

 seems, according to the general consent of all experimenters on the 

 mammalian heart, that it is much more easily possible to obtain 

 standstill of the auricles with the ventricle still beating than in the 

 case of the cold-blooded animal. This is said to be of special frequent 

 occurrence on stimulation of the vagus. MacWilliam, 2 Koy and 

 Adami, 3 Krehl and Eoniberg, 4 and others, have all noticed cases where 

 vagus stimulation has caused cessation of auricular beats, while the 

 ventricle has continued to beat, though with altered rhythm. It is 

 said, in fact, that stimulation of the vagus has brought to light the 

 independent ventricular rhythmical beat, which could not manifest 

 itself previously owing to the greater rapidity of the veno-auricular 

 beat. MacWilliam here gives the same warning as I have already 

 given, as to the difficulty of being sure that the standstill of the 

 auricles is absolute ; and he expressly mentions cases where his curves 

 show complete cessation of auricular contractions, although upon close 

 inspection he convinced himself that the auricles were still beating 

 very feebly. The fact, however, that an independent ventricular rhythm 

 can be unmasked by a vagus stimulation, which causes standstill of the 

 auricles, is shown most clearly by an example given by me in my paper 

 on the tortoise heart (Fig. 8, Plate IV.), where an independent ventricular 

 rhythm, with a reversed sequence of auriculo-ventricular contraction, was 

 unmasked at every stimulation of the vagus which caused sino- auricular 

 standstill. The development of this ventricular rhythm could be seen, 

 by means of a series of stimulations of the vagus nerve, to take place in 

 the same way as its development in an isolated ventricle ; and its com- 

 plete independence of the auricles and sinus was seen at the end of the 

 experiment by simply cutting away the auricles ; the ventricle then 

 immediately went on beating at the rate it had been shown to have 

 attained at the last stimulation of the vagus. In the case of the 

 mammalian heart, this last decisive proof of the unmasked ventricular 

 rhythm being truly independent, is unfortunately wanting. 



The peculiarities of the cardiac muscle by which the rhythm 

 and sequence may be explained. — As is well known, the cardiac 

 muscle, though striated, differs in respect to stimulation from the 

 ordinary striated skeletal muscle in various important points, of which 

 the principal are : — 



1 "The Nutrition of the Heart through the Vessels of Thebesius and the Coronary 

 Veins," Am. Journ. Physiol., 1898, vol. i. p. 86. 



2 Journ. Physiol., Cambridge and London, 1888, vol. ix. pp. 167, 345. 



3 Phil. Trans., London, 1892. 



4 Op. cit., 1892. 



