CARDIAC INHIBITION AND ACCELERATION 315 



more easily, the augmentor effect cannot develop until the inhibition 

 has been brought to a close, or has lost its initial power. In mammals, 

 such as the dog, cat, and rabbit, the effects of the stimulation of the 

 vagus are very similar to those noted in the frog and turtle, with this 

 exception, however, that the secondary augmentation is less pro- 

 nounced. It also seems that in these animals the augmentor and 

 the inhibitor fibers antagonize one another in a direct manner, because 

 the excitation of the former tends to lessen the action of the latter. 



The Nature of the Inhibition. — Before entering upon a discussion 

 of this topic, brief reference should be made to the question of whether 

 the vagal impulses are distributed solely to the auricles or to the 

 ventricles, or to both parts. Thi;is, it may be held, on the one hand, that 

 their influence is apportioned equally to all parts of the organ 

 and, on the other, that it is distributed solely to the auricle and par- 

 ticularly to the area of the ''pace-maker." In the first instance, 

 therefore, the cardiac musculature would be affected directly and, in 

 the second, solely through the intervention of the sino-auricular node. 

 The latter view necessitates the assumption that the ventricle is ren- 

 dered inactive on account of the failure of the ''pace-maker" to dis- 

 charge those waves of excitation which ordinarily give rise to its 

 activity. Gaskell has submitted certain evidence to show that in 

 the terrapin the inhibitor impulses are received by the auricle, and that 

 the ventricle ceases to beat because no stimuli are apportioned to it 

 by the "pace-maker." In the frog, on the other hand, the ventricle 

 is under the direct control of the vagus, quite independently of the 

 auricles. A similar relationship seems to exist in the mammals, 

 the vagus fibers being distributed to the auricles as well as to the ven- 

 tricles. ^ This is shown by the fact that the contractions of the auricular 

 and ventricular musculature may be dissociated and even reversed. 



The vagal impulses produce their characteristic effect either by 

 causing the musculature to relax, or by diminishing the power of 

 conduction of the bundle of His and its ramifications. On the whole, 

 however, the experimental evidence favors the view that the vagus 

 exerts its action primarily through the auricles and the "pace-maker" 

 and that its direct action upon the ventricles is slight and is made use 

 of only under singular circumstances. Engelman^ classifies the cardio- 

 motor impulses as follows : 



(a) Chronotropic, affecting the rate of the contractions. 

 (6) Inotropic, affecting the force of the contractions. 



(c) Bathmotropic, affecting the irritabiUty of the muscular tissue, and 



(d) Dromotropic, affecting the conductivity of the tissue. 



Every one of these influences is said to be either of a positive or 

 negative kind. The former result in consequence of the excitation 

 of the accelerator, and the latter in consequence of the stimulation of 



1 Tigerstedt (Lehrb. der Physiol, des Kreisl., Leipzig, 1893), Frank (Arch, 

 der Physiol., 1891), and McWiUiams (Jour, of Physiol., ix, 1888). 



2 Arch, fiir Physiol., 1900 and 1902. 



