154 AX AMERICAN TEXT-BOOK OE PHYSIOEOGY. 



Although the normal course of the excitation-wave is from base to apex, it 

 can be made to travel in any direction. If the frog's ventricle is cut with fine 

 scissors into a number of pieces in such a way as to leave small bridges of 

 heart-tissue between each piece, and any one of the pieces is stimulated, the 

 contraction will begin in the stimulated piece and then run from piece to piece 

 over the connecting bridges until all have successively contracted. The direc- 

 tion in which the excitation-wave travels can thus be altered at the pleasure 

 of the operator. 



Whether the excitation is propagated from mnscle-cell to muscle-cell or by 

 means of nerve-fibres has given rise to much discussion. Anatomical evidence 

 can be adduced on both sides. On the one hand the rich plexus of nerve- 

 fibres everywhere present in the heart-muscle suggests conduction through 

 nerves ; on the other is the intimate contact of neighboring muscle-cells over 

 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. 



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. 1 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. 



It is possible to cut the ventricular muscle in a zigzag or spiral fashion 

 that makes probable the severance of all the nerve-fibres in the line of the 

 cut, and yet the contraction will pass from one end to the other of the isolated 

 strip. 2 



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 



1 Engelmann : Archivfiir die gesammte Physiologie, 1896, lxii. p. 549. 



2 I'urter: American Journal of Physiology, 1899, ii. p. 127. The co-ordination of the ven- 

 tricles is discussed in this paper, and also by von Vintschgau : Archiv fur die gesammte Physi- 

 ologie, 1899, lxxvi. p. 59. 



