198 THE VASCULAR MECHANISM. 



induced by motor impulses reaching it along its nerve, does not hold 

 good. 



These and other considerations, taken together with the facts already 

 mentioned, that portions of cardiac muscular tissue in which ganglionic cells 

 are certainly not present, can in various animals be induced, either easily or 

 with difficulty, to execute rhythmic beats which have all the appearance of 

 being spontaneous in nature, lead us to conclude that the beat of the heart 

 is not the result of rhythmic impulses proceeding from the cells of the ganglia 

 to passive muscular fibres, but is mainly the result of changes taking place 

 in the muscular tissue itself. And here we may call attention to the peculiar 

 histological features of cardiac muscular tissue ; though so far differentiated 

 as to be striated, its cellular constitution and its " protoplasmic " features, 

 including the obscurity of the striation, show that the differentiation is in- 

 complete. Now, one attribute of undifferentiated primordial protoplasm is 

 the power of spontaneous movement. 



142. We have, moreover, evidence that it is the muscular tissue, and 

 not the arrangement of ganglia and nerves, which is primarily concerned in 

 maintaining the remarkable sequence of sinus beat, auricle beat, and ven- 

 tricle beat. This is perhaps better seen in the heart of the tortoise than in 

 that of the frog. 



In this animal the nerves passing from the sinus to the ventricle may be 

 divided, or the several ganglia may be respectively removed, and yet the 

 normal sequence is maintained. On the other hand, we find that interference 

 with the muscular substance of the auricle, when carried to a certain extent, 

 prevents the beat of the auricle passing over to the ventricle, so that the 

 sequence is broken after the auricle beat. If, for instance, the auricle be cut 

 through until only a narrow bridge of muscle be left connecting the part of 

 the auricle adjoining the sinus with the part adjoining the auriculo-ventric- 

 ular ring, or if this part be compressed with a clamp, a state of things may 

 be brought about in which every second beat only, or every third beat only, 

 of the sinus and auricle is followed by a beat of the ventricle ; and then, if 

 the bridge be still further narrowed or the clamp screwed tighter, the ven- 

 tricle does not at all follow in its beat the sequence of sinus and auricle, 

 though it may after a while set up an independent rhythm of its own. 

 This experiment suggests, and other facts support, the view that the normal 

 sequence is maintained as follows: The beat begins in the sinus (including 

 the ends of the veins) ; the contraction wave, beginning at the ends of the 

 veins, travels over the muscular tissue of the sinus, and reaching the auricle 

 starts a contraction in that segment of the heart ; similarly the contraction 

 wave of the auricular beat reaching the ventricle starts a ventricular beat, 

 which in turn in like fashion starts the beat of the bulbus. And in hearts 

 in a certain condition it is possible by stimulation to reverse this sequence, 

 or to produce, by alternate stimulation, an alternation of a normal and a 

 reversed sequence ; thus in the heart of the skate, in a certain condition, 

 mechanical stimulation of the bulbus by indicating a beat of the bulbus 

 will start a sequence of the bulbus. ventricle, auricle, and sinus, and similar 

 stimulation of the sinus will produce a normal sequence of sinus, auricle, 

 ventricle, and bulbus. 



It would, perhaps, be premature to insist that the nervous elements do- 

 not intervene in any way in the maintenance of this sequence ; but the evi- 

 dence shows that they are not the main factors, and we have at present no 

 satisfactory indications of the way in which they do or may intervene. 



Two questions naturally suggest themselves here. The first is, Why does 

 the cardiac cycle begin with the sinus beat? We have previously ( 141) 

 given the evidence that the sinus has a greater potentiality of beating than 



