1857.] as to the Structure and Functions of Nerve. 433 



traction of these chambers the blood is propelled in a certain direc- 

 tion ; the auricles contracting force the blood into the ventricle ; 

 the ventricle then contracting drives the blood into the aortic bulb ; 

 and it is essential to the full efficiency of the heart as a circulatory 

 organ, that all the muscular fibres of the auricles should contract 

 together ; and that all the muscular fibres of the ventricle should 

 contract together ; but that the latter should follow the former 

 action after a certain interval. 



The contractions of the muscles of the heart thus occur in a 

 definite order, and exhibit a combination towards a certain end. 

 They are rhythmical and purposive ; and it becomes a question of 

 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 guid- 

 ing the movements of the heart. The muscular tissue, on the 

 other hand, 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 beat- 

 ing ; 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 pul- 

 sating, 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 ventricles. 



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 



Vol. II. 2 H 



