I 4 2 THE CIRCULATION OF THE BLOOD AND LYMPH 



automatism i.e., its power of beating in the absence of external 

 stimuli; (2) its rhythmicity i.e., its power of responding to con- 

 tinuous stimulation by a series of rhythmically repeated contrac- 

 tions; (3) its conductivity i.e., its power of conducting the contrac- 

 tion wave or the impulse to contraction once it has been set up ; and 

 (4) the power of co-ordination, in virtue of which the various parts 

 of the heart beat in a regular sequence. 



The excitability of the cardiac tissue that is, its power of appro- 

 priate response (namely, by contraction) to a suitable stimulus 

 does not particularly concern us here, since it is in no wise a property 

 special to the heart. Only, as we shall see in the sequel, the time- 

 relations of this excitability are of interest, for the existence of a 

 refractory period that is, an interval during which the cardiac 

 muscle refuses to respond to excitation throws light upon the 

 rhythmicity of the heart-beat. The tonicity of the heart i.e., its 

 power of remaining contracted to a certain extent in the intervals 

 between successive beats is another property of great importance 

 in certain aspects, but which only needs to be mentioned at present. 



Automatism of the Heart-Beat Neurogenic and Myogenic Hypo- 

 theses. That the heart-beat is automatic is sufficiently shown by 

 the fact that, as already mentioned, an excised and empty heart 

 will go on beating for a time, for many hours or even for days in the 

 case of cold-blooded animals. When blood, or even a suitable 

 solution of such inorganic salts as exist in serum, is caused to circu- 

 late through the coronary vessels of the excised heart of a warm- 

 blooded animal, it also continues to contract for a long time. In 

 trying to understand the real significance of the automatic beat of 

 the heart, physiologists have endeavoured, first, to compare different 

 portions of the heart as regards the degree in which they possess this 

 property of automaticity ; and, second, to associate, if possible, one 

 or other of the active tissues that compose the organ, muscle, and 

 nervous tissue, with this characteristic property. It cannot be 

 pretended that a final answer to this question is possible at present. 

 Nor is the historical controversy which it has occasioned perhaps as 

 important in itself as the space usually devoted to it in textbooks 

 might imply. Yet it is probable that the series of fundamental 

 facts in the physiology of the heart elicited in the long discussion can 

 be best presented, even for the purposes of the elementary student, 

 as they were originally brought forward in the form of pros and 

 cons, of arguments for and against the neurogenic or the myogenic 

 hypothesis. There is good evidence that as in the amphibian 

 heart the contraction starts in the sinus venosus, so in the mam- 

 malian heart it starts in the sinus tissue of the right auricle in the 

 region of the sino-auricular node. Attempts have been made to 

 demonstrate that the origination of the impulses which are after- 

 wards conducted to all parts of the heart is normally confined to 



