CIRCULATION. 447 



the nervous conduction as the delay which the excitation experiences in dis- 

 charging the ganglion-cells of the ventricle, in accordance with the well-known 

 hypotheses of the retardation of the nerve-impulse in sympathetic ganglia 

 and the slow passage of the nervous impulse through spinal cells. 



The explanation given by those who believe in muscular conduction is that 

 the small number of muscular fibres composing the bridge between auricle 

 and ventricle acts as a " block " to the excitation-wave. If the auricle of the 

 tortoise heart is cut into two pieces connected by a small bridge of auricular 

 tissue, the stimulation of one piece will be followed immediately by the con- 

 traction of that piece, and after an interval by the contraction of the other. 

 The smaller the bridge, the longer the interval; that is the longer the excita- 

 tion-wave will be in passing from one piece to another. 1 



The duration of the pause or " block " in the frog has been found to be from 

 0.15 to 0.30 second. The length of the muscle-fibres connecting auricle and 

 ventricle is about one millimeter. The speed of the excitation- wave in em- 

 bryonic heart muscle is from 3.6 to 11.5 millimeters per second. The duration 

 of the pause agrees, therefore, with the time which would be required for 

 muscular conduction. 2 



The extensive extirpations of the auricular nerves which have been made 

 without stopping conduction from auricle to ventricle 3 for example, the ex- 

 tirpation of the entire auricular septum of the frog's heart are of little 

 importance to this question, since the great number of nerve-cells revealed by 

 recent methods make it improbable that any extirpation short of total removal 

 of both auricles could cut off all the nerve-cells of the auricle. 



Refractory Period and Compensatory Pause. Schiff 4 found in 1850 

 that the heart which contracted to each stimulus of a series of slowly repeated 

 mechanical stimuli would not contract to the same stimuli if they followed each 

 other in too rapid succession. Kronecker 5 got a similar result with induction 

 shocks. The heart contracted to every stimulus only when the interval between 

 them was not too brief. The following year Marey 6 published a systematic 

 study of the phenomenon. He observed that the irritability of the heart sank 

 during a part of the systole, but returned during the remainder of the systole 

 and the following diastole. 7 The stimulus which fell between the beginning of 

 the systole and its maximum produced no extra contraction, whilst that which 

 fell between the maximum of one systole and the beginning of the next 

 called forth an extra contraction. During a part of the cardiac cycle therefore 

 the heart is " refractory" toward stimuli. The irritability of the heart is 

 removed for a time by an adequate stimulus. 



Kronecker and Marey noticed further that stimulation with the induction 

 shock during the non-refractory period did not influence the total number of 

 systoles. The extra systole called forth by the artificial stimulus was followed 

 by a pause the length of which was that of the normal pause plus the interval 



1 Gaskell, 1883, p. 64. 2 Engelmann, 1894, p. 159. 



3 Gaskell, 1883, p. 75 ; Hoffmann, 1895, p. 169. 4 Schiff, 1850, p. 50. 



5 Kronecker, 1875, p. 181. 6 Marey, 1876, p. 73. 7 Cf. Engelmann, 1895, p. 313. 



