288 MAINTENANCE OF SEQUENCE. [BOOK i. 



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 stria- 

 tion, shew that the differentiation is incomplete. Now one attribute 

 of undifferentiated primordial protoplasm is the power of spon- 

 taneous movement. 



156. 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 ventricle 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 main- 

 tained. 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-ventricular 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 ventricle 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 the 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 

 inducing a beat of the bulbus will start a sequence of 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 evidence shews that they are not the main factors, 



