572 CIRCULATION OF BLOOD AND LYMPH. 



developed at the venous end, according to recent views in that 

 remnant of sinus tissue known as the sino-auricular node. This 

 portion of the heart, therefore, contracts first and the wave of 

 contraction spreads directly to the musculature of the auricle and 

 thence through the auriculo-ventricular bundle to that of the 

 ventricle. The quickly beating venous end sets the pace, as it 

 were, for the entire heart. The nerve cells and nerve-fibers that 

 are present in the heart are upon this theory supposed to be con- 

 nected with the extrinsic nerves through which the rate and force 

 of the heart beat are regulated, but they are not concerned in the 

 production of the beat. Many experimental facts have been ac- 

 cumulated which give probability to this view, and it has been 

 adopted by many, perhaps most, of the recent workers in this 

 field. Some of the facts that favor this theory are as follows: 



1. The anatomical arrangement of the musculature of the 

 heart is not opposed to such a theory. It was formerly stated quite 

 positively that there is no muscular connection between the auricles 

 and ventricles in the mammalian heart, but we now know that 

 these two parts of the heart are connected through a peculiar 

 system of muscular tissue, the auriculoventricular bundle and its 

 ramifications. It may be accepted also that the wave of excitation 

 from the sinus end of the heart passes along this system. All the 

 detectable nerve trunks crossing the auriculoventricular groove 

 may be cut without altering the sequence of the heart beat, but 

 section or compression of the A-V bundle brings on at once the 

 condition of dissociated heart-rhythm known as heart block. 

 The auriculoventricular bundle contains nerve-fibers as well as 

 muscle-fibers, and the advocates of the neurogenic hypothesis 

 make, therefore, the somewhat improbable claim that these par- 

 ticular nerve-fibers of all those that pass between auricle and ven- 

 tricle are the only ones concerned in the conduction of the normal 

 stimulus from auricle to ventricle. 



2. The fact that a contraction started at one part of the heart 

 may travel to other portions through the intervening musculature 

 may be said to be demonstrated. Thus, Engelmann has shown 

 that if the ventricle in the frog's heart is cut in a zigzag fashion, 

 so that strips are obtained which are connected only by narrow 

 bridges, a stimulation applied at one end starts a wave of con- 

 traction which propagates itself over all of the pieces. This and 

 similar experiments scarcely permit of explanation on the supposi- 

 tion that conduction from piece to piece is effected by a definite 

 nervous mechanism. So too it has been shown that under certain 

 conditions the normal auriculo-ventricular rhythm can be changed 

 at will to a ventriculo-auricular rhythm. If, for instance, a ligature 

 be tied around the frog's heart between the sinus venosus and the 



