300 COHN— STUDY OF THE [April 23. 



the heart. An explanation of the nature of these abnormal curves 

 is obtained by comparing them with others from hearts to which 

 direct mechanical stimuli have been applied. A stimulus to the left 

 side of the heart (ventricle) yields a curve like that seen when the 

 right branch of the conducting bundle is cut, while one applied to 

 the right side is like the curve seen when the left' tract is cut. Under 

 both conditions, contraction is initiated at the site of the stimulus, 

 and the impulse spreads over the heart from this area. The form 

 of the curve yielded depends on whether the right or left ventricle 

 initiates the contraction. 



The conduction bundle, then, is an important factor in the orderly 

 cardiac mechanism, but, on account of its exposed position in the 

 heart, it is frequently injured. The pathway between the auricles 

 and the ventricles is, therefore, interrupted and impulses from one to 

 the other are consequently blocked. The ventricles, deprived of 

 stimuli from above, contract independently and without reference to 

 the rate or rhythm of the auricular beats. In the study of derange- 

 ments of the cardiac mechanism of this nature, electrocardiography 

 has rendered distinct service. 



So far we have discussed the mechanism of the heart-beat only in 

 so far as it relates to the intrinsic arrangements of the heart. But 

 for the proper exercise of a number of its functions, the heart is sub- 

 jected to the influence of the central nervous system. In the study 

 of this influence the galvanometer has been not only useful, but 

 essential. Branches from the central nervous system to the heart 

 exercise functions of two sorts, — inhibitory and accelerator. It has 

 been especially our work to show that the inhibitory or slowing function 

 is not simple and is not possessed equally by both vagus nerves. To say 

 that the right vagus nerve principally modifies the rate of the heart 

 and the left vagus chiefly conduction between its chambers states a 

 truth and also indicates the presence of a complicated mechanism 

 which may be explained in the following way : The complexity de- 

 pends on the fact that the heart was originally an unpaired organ 

 with a symmetrically distributed nerve supply. This supply, so far 

 as we are informed, was directed to the junction between the old 

 sinus venosus and the auricles. In the development of the heart, 

 a division of the junctional tissue took place; one portion, the right. 



