3i6 ON thp: present state of knowledge 



extreme interest to ascertain, where lies the regulative power which 

 governs their rhythm. 



If we examine into the various structures of which the heart is 

 composed, we find that the bulk of the organ is made up of striped 

 muscular fibres, bound together as it were by connective tissue, and 

 lined internally and externally by epithelium. Now it is certain that 

 the regulative power is not to be found in any of these tissues. 

 The two latter may, for the present purpose, be regarded as unim- 

 portant, as they certainly take no share either in producing or guiding 

 the movements of the heart. The muscular tissue, on the other liand, 

 though the seat of the contractility of the organ, requires some 

 influence from without, some stimulus, in order to contract at all, and 

 having once contracted, it remains still until another stimulus excites 

 it. There is, therefore, nothing in its muscular substance which can 

 account for the constantly recurring rhythmical pulsations of the 

 heart. 



Experiments have been made, however, which clearly show that 

 the regulative power is seated, not only in the heart itself, but in 

 definite regions of the organ. Remove the heart from the body, and 

 it still goes on beating ; the source of the rhythm is therefore to be 

 sought in itself If the heart be halved by a longitudinal section, each 

 half goes on beating ; but if it be divided transversely, between the 

 line of junction of the auricles with the ventricle and the apex of the 

 latter, the detached apex pulsates no longer, while the other segment 

 goes on beating as before. If the section be carried transversely 

 through the auricles, both segments go on beating ; and if the heart 

 be cut into three portions by two transverse sections, one above the 

 junction of the auricles and ventricle, and one below it, then the 

 basal and middle segments will go on pulsating, while the apical 

 segment is still. Clearly then, the source of the rhythmical action, 

 the regulative power, is to be sought somewhere about the base of 

 the auricles, and somewhere about the junction of the auricles and 

 \entricles. 



Now there is in the frog's heart, besides the three tissues which 

 have been mentioned, a fourth, the nervous tissue. A ganglion is 

 placed at the base of the heart, where the great veins enter the 

 auricles — from this two cords can be traced traversing the auricular 

 septum, and entering two other ganglia placed close to the junction 

 of the auricles with the ventricles. From these ganglia nerves are 

 distributed to the muscular substance. Now we know, from evidence 

 afforded by other striped muscles and nerves, that the contraction of 

 the former is the result of the excitement of the latter ; in like 



