CIR CULA TION. 1 55 



near the auricula- ventricular junction. The controversy over the nervous or 

 muscular conduction of the excitation within the auricle and ventricle has 

 been extended to its passage from auricle to ventricle. A path for conduction 

 by nerves is presented by the numerous nerves which go from the auricle to 

 the ventricle. It has been shown recently that muscular connections also 

 exist. In the frog, muscle-bundles pass from the auricle to the ventricle 

 where the auricular septum adjoins the base of the ventricle. Muscular 

 bridges pass also from the sinus venosus to the auricles and from the ventricle 

 to the bulbus arteriosus. 1 These muscle-fibres appear to be in intimate con- 

 tact with the muscle-cells of the divisions of the heart which they unite. Gas- 

 kell believes that the connecting fibres are morphologically and physiologically 

 related to embryonic muscle, and therefore possess the power of contracting 

 rhythmically. 



The delay experienced by the excitation in its passage from the auricle to 

 the ventricle — in other words, the normal interval between the contraction of 

 the auricle and the contraction of the ventricle — is explained by those favoring 

 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. 



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 8 — 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 aerve-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. 



It is possible to explain the occurrence of intermittent or irregular eon- 



1 Engelmann: Archiv fur die geaammUe Physiologic, 1894, lvi. j>. 158. 

 ' Engelmann : Tbid., p. 159. 

 •Hofinann: Ibid., L895, lx. j>. 169. 



