244 FEATURES OF CARDIAC CONTRACTION. [BOOK i. 



skeletal muscle in the following important feature. When we 

 stimulate a skeletal muscle with a strong stimulus we get a large 

 contraction, when we apply a weak stimulus we get a small 

 contraction ; within certain limits (see 74) the contraction is 

 proportional to the stimulus. This is not the case with the qui- 

 escent ventricle or heart. When we apply a strong induction- 

 shock we get a beat of a certain strength ; if we now apply a 

 weak shock we get either no beat at all or quite as strong a beat 

 as with a stronger stimulus. That is to say the magnitude of 

 the beat depends on the condition of the ventricle (or heart) and 

 not on the magnitude of the stimulus. If the stimulus can stir 

 the ventricle up to beat at all, the beat is the best which the 

 ventricle can at the time accomplish ; the stimulus produces 

 either its maximum effect or none at all. It would seem as if 

 the stimulus does not produce a contraction in the same way 

 that it does when it is brought to bear on a skeletal muscle, but 

 rather stirs up the heart in such a way as to enable it to execute 

 a spontaneous beat which, without the extra stimulus, it could 

 not bring about. And we have reason to think that the normal 

 beat of the heart within the body is the maximum beat of which 

 it is capable at the moment. These and other special features of 

 the contraction of cardiac muscle lead to the conclusion that the 

 rhythmic power does not reside wholly in the ganglia ; but we 

 must not here discuss the question further, nor enter upon the 

 difficult problem of how the remarkable sequence in contraction 

 of the several parts is developed and as a rule maintained. 



134. In the above we have dealt chiefly with the heart of 

 the cold blooded animal, but so far as we know the same general 

 conclusions hold good for the mammalian heart also. There is, it 

 is true, in the mammal, no prepotent sinus venosus, but as in the 

 frog the auricles are dominant, and their beat determines the beat 

 of the ventricles. Even more clearly than in the frog however, 

 the ventricles, though they normally follow the auricles in their 

 beat, being initiated as it were by them, possess an independent 

 rhythmic power of their own. By a mechanical contrivance all 

 conduction of nervous or muscular impulses between the auricles 

 and ventricles may be abolished, though the blood may continue to 

 flow from the cavities of the former to those of the latter. When 

 this is done the ventricles go on beating rhythmically, but at a 

 rate which is quite independent of that of the auricular beats. 



We may now turn to the nervous mechanisms by which the 

 beat of the heart, thus arising spontaneously within the tissues of 

 the heart itself, is modified and regulated to meet the require- 

 ments of the rest of the body. 



The Government of the Heart Beat by the Nervous System. 



135. It will be convenient to begin with the heart of the 

 frog. This is connected with the central nervous system through, 



