ix CARDIAC MUSCLE AND NERVES 287 



between the ventricle and the bulbus arteriosus. It therefore 

 follows that when the auricles begin to contract, the sinus begins 

 to expand ; when the ventricle enters systole, the auricles are 

 commencing diastole ; when, lastly, the systole of the bulbus 

 arteriosus begins, the ventricle is entering diastole. The two 

 active phases (systole and diastole) are followed by the state of 

 rest or functional pause, which in the same way appears earlier 

 in the sinus than in the auricles, and in the auricles than in the 

 ventricle, with the resting phase of which the cardiac cycle or 

 revolution is complete. 



These phenomena, which are easily detected on the frog or the 

 tortoise, are in complete agreement with those which can be observed 

 under greater difficulties in warm-blooded animals. In these, 

 too (as we have seen), the wave of contraction arises in the 

 large veins, which, as they have no sinus venosus, open directly 

 into the auricle : from the auricles it is propagated peristaltically 

 to the ventricles, where it is arrested, since these have no contrac- 

 tile bulbi arteriosi. In warm-blooded animals, too, the wave of 

 contraction encounters a block, or momentary delay, at the passage 

 from auricles to ventricles, so that the diastole of the auricles 

 coincides with the commencement of ventricular systole, a neces- 

 sary condition for the perfect functioning of the cardiac pump. 



From these fundamental phenomena arise all the complex 

 problems relating to the physiological conditions of the cardiac 

 functions. On what does the rhythmical action of the heart 

 depend ? What are the external chemical conditions indispensable 

 to its activity ? Is its rhythm of a reflex nature, dependent on 

 extrinsic conditions or stimuli, or is it automatic in character, 

 dependent on intrinsic conditions or stimuli within the heart ? 

 Is it a function of the nervous system, or simply a property per- 

 taining to the cells of cardiac muscle ? Is the peristaltic contraction 

 wave propagated by way of the nerves or by the muscle cells ? 

 Why does the wave of contraction arise in the venous paths that 

 lead to the heart ; and on what do the blocks or brief delays, to 

 which it is subjected on passing from one segment of the heart to 

 the other, and which are of such great importance to its mechanical 

 functions, depend ? These are the fundamental problems which 

 have to be examined. 



METHODS OF STUDYING CARDIAC MOVEMENT 



Cardiac movement must be studied either on the exposed heart in situ, or on 

 the fully isolated heart. The frog's heart isolated from the body is capable, 

 provided it be protected from drying in a moist chamber, of continuing its 

 normal activity for some time (several days), and thus represents the most 

 accessible object for the study of cardiac movements. For this reason the 

 methods employed in the graphic registration of these movements were 

 especially contrived for the frog's heart. Observations of the surviving heart 



