3 io TEXT-BOOK OF PHYSIOLOGY. 



and vagal fibers, to the heart, and which, in a manner, as yet unexplained 

 accelerate or augment or inhibit, the action of the heart. The relation which 

 these two centers bear one to the other and the manner in which they are 

 influenced in their activities both directly and reflexly and thus regulate the 

 action of the heart from moment to moment will be considered in a subse- 

 quent paragraph. 



Changes in the Conductivity of the Heart. In addition to the 

 changes in the rate and force of the heart caused by stimulation of the inhib- 

 itor and the augmentor nerves, it is stated by Gaskell that there is also during 

 the inhibition a decrease in the conductivity of the heart at both the sino- 

 auricular and auriculo-ventricular junctions, and an increase in the con- 

 ductivity during acceleration of the beat. The decrease in conductivity 

 may be so pronounced that only every second or third contraction of the 

 auricle will be followed by a contraction of the ventricle. In other instances 

 both auricles and ventricles remain at rest while the sinus maintains its usual 

 rate. The increase in conductivity is shown by first artificially blocking the 

 contraction wave at the auriculo-ventricular junction with the clamp, until 

 only every second or third auricular contraction is conducted to the ventricle, 

 and then stimulating the sympathetic. At once the auricular contraction 

 forces the block, and passes to the ventricle, calling forth a normal contraction 



The Physiologic Actions of the Sympathetic Nerves in Mammals. 

 In the mammal, stimulation of the sympathetic nerves in any part of their 

 course, either through the rami communicantes, the ventral portion of the 

 annulus of Vieussens, or after their emergence from the stellate or inferior 



FIG. 145. ACCELERATION OF THE HEART FOLLOWING STIMULATION OF THE CARDIAC 

 BRANCHES WHICH COME FROM THE ANNULUS OF VIEUSSENS. 



cervical ganglia is followed by effects similar to those observed in the frog: 

 viz., an acceleration or augmentation, or both, of the heart-beat. The 

 percentage increase in the acceleration varies in different animals. In some 

 instances the increase varies from 58 per cent, to 100 per cent. (Hunt). If 

 the heart is beating slowly before stimulation, the acceleration is more marked 

 than if it is beating rapidly. 



The effect of the accelerator impulses is apparently a change in the inner 

 mechanism of the heart-muscle itself and not a change in the peripheral 

 portion of the inhibitor apparatus. This is indicated by the fact that 

 acceleration occurs after the full physiologic action of atropin, which acts 

 upon, and impairs the conductivity of, the intra-cardiac nerve-cell terminals. 



A peculiarity of the sympathetic nerve is that it does not respond to 

 stimulation as rapidly as do many nerves, so that a rather long latent period 

 intervenes between the moment of stimulation and the appearance of the 

 acceleration as shown in Fig. 145. A further peculiarity is that the accelera- 

 tion sometimes continues after the stimulus is withdrawn, and sometimes 

 ceases before it is withdrawn. 



Though an increase in both the rate and force frequently occur simul- 



