446 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



a part at least of their surface, thus bringing one mass of irritable protoplasm 

 against another and offering a path by which the excitation might travel from 

 cell to cell. 1 



If the excitation-wave were conducted by means of nerves, the difference 

 between the moment of contraction of the ventricle when the auricle is stimu- 

 lated near the ventricle, and again as far as possible from the ventricle, should 

 be very slight, because of the great speed at which the nervous impulse travels 

 (about 33 meters per second). If, on the contrary, the conduction were by 

 means of muscle, the difference would be relatively much greater, correspond- 

 ing to the much slower conductivity of muscular tissue. It has been found by 

 Engelmann that the ventricle contracts later when the auricle is stimulated far 

 from the ventricle than when it is stimulated near the ventricle. The rate of 

 propagation being calculated from the difference in the time of ventricular con- 

 traction was found to be 90 millimeters per second, which is about 300 times 

 less than the rate which would have been obtained had conduction over the 

 measured distance taken place through nerves. 2 Hence the stimulus that trav- 

 els through the auricle to the ventricle and causes its contraction should be 

 propagated in the auricle by muscle-fibres and not by nerves. 



Passage of Excitation-wave from Auricle to Ventricle. The normal con- 

 traction of the heart begins, as has been said, at the junction of the great 

 veins and the auricle, spreads rapidly over the auricle and, after a distinct 

 pause, reaches the ventricle. The normal excitation-wave preceding the con- 

 traction passes likewise from the auricle to the ventricle and is delayed at or 

 near the auriculo- 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. 3 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. 4 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 5 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 



1 Engelmann, 1874, p. 7. 



2 Ibid., 1894, p. 188 ; 1896, p. 549; the measurements of Bayliss and Starling, 1892, p. 271, 

 on the mammalian heart are probably of little value because of the variation due to tempera- 

 ture (p. 272). See also Kaiser, 1895, p. 2, and Engelmann's reply, 1896, p. 547. 



3 Paladino, 1876 ; Gaskell, 1880, p. 70 ; Krehl and Romberg, 1892, p. 71 ; Kent, 1893, p. 

 240 ; Engelmann, 1894, p. 158. 



* Engelmann, 1894, p. 158. * Gaskell, 1883, p. 77. 



