THE CIRCULATION. 175 



Physiologists have, therefore, been driven to the belief that 

 the heart muscle itself has a kind of automaticity which 

 helps to produce the rhythmic beat. 



That these muscle cells themselves must be to some ex- 

 tent automatic may be further conclusively shown by cut- 

 ting out a strip of muscle from a terrapin's heart, which has 

 no ganglia in it, and observing that such a strip will continue 

 to beat rhythmically if it is once started to do so. Nervous 

 influence is here again entirely out of the question. There 

 are still other reasons to think that heart muscle is auto- 

 matic and determines its own rhythm. Thus, the heart mus- 

 cle cannot be thrown into a tetanic contraction, no matter 

 how rapid the stimuli. If the heart contracts at all it will 

 beat with its own rhythm. Then again, the contraction of 

 the heart does not vary with the strength of the stimulus. 

 A slight stimulus will not produce a slight contraction, and 

 a stronger stimulus a stronger contraction, as in the case of 

 all other muscles; but if the stimulus acts at all, be it strong 

 or weak, it will produce the natural rhythmic beat. 



What has been said so far with reference to the innerva- 

 tion of the heart has been observed on the lower animals, 

 but there is every reason to believe that practically the 

 same conditions exist in the higher animals. By way of 

 summary be it stated what the condition of things prob 

 ably is in the human heart. Near the mouths of the veins 

 entering the auricles are groups of ganglia called here, also, 

 the ganglia of Remak. A second group of ganglia occurs 

 in the septum between the auricles at the place where this 

 touches the ventricles; that is, in the walls between the 

 two auriculo- ventricular valves. But unlike the frog, scat- 

 tered ganglia are found through the apex of the heart. As 

 in the case of all animals where observations have been 

 made, the impulse to beat in the human heart is intrinsic 

 and is due probably to two things. First, the ganglia 

 themselves in the heart are probably automatic. Second, 

 the heart muscles as well possess an automaticity, and the 

 beat is the result of these two similarly directed influences. 



