296 THE CIRCULATION 



If the vagus be cut within the skull of an animal (amphibian or mam- 

 mal), and the peripheral stump be stimulated with an induced electrical 

 current, the result is inhibition, more or less, of the heart. The details of 

 this effect 'depend not only on the strength of the stimulus, but on the 

 condition, as well as the genus, of the animal. A latent period is first 

 obvious, then there occurs a slowing of the beat (perhaps a stopping) 

 and a decrease in the force of the contractions. If the stimulation last 

 only a few seconds, the inhibition is continued meanwhile and for a 

 longer or a shorter time after the former is stopped. Immediately 

 afterward the heart gradually increases the force (extent) of its contrac- 

 tions until they may far exceed their amplitude and rate before the 

 stimulation. If the excitation be long-continued, it is soon obvious that 

 the vagus has lost control of the heart, for despite its influence the diastoles 

 shorten, the contractions increase in force, and the heart is soon beating 

 almost as if the vagus were not conducting inhibitory impulses to it. 

 This last phenomenon is due perhaps to the rhythmicity which is almost 

 part of the muscle-cells of the heart, and perhaps due to resident 

 ganglia. It shows that nothing short of destruction of tissue can long 

 impede the beating. The secondary augmentation of the beat (occur- 

 ring after the inhibition) is much more conspicuous in the frog, etc., 

 than in the mammal, and so far is the influence of the vagus over the 

 ventricles. Indeed, in the mammal this latter effect is often quite inap- 

 preciable. 



There is plenty of evidence that both of these sets of nerves are con- 

 ducting regulating impulses to the heart continually and that the way 

 the viscus works is in a measure the result of the balance between the 

 two influences, augmentor and inhibitory. Removal of the inhibitory 

 factor by the cutting of both vagi alone allows the augmentor to increase 

 the rate of the heart-beat. On the contrary, cutting of both vagi and 

 removal of the two ganglia in the course of the augmentor fibers from 

 the cord to the heart (the stellate and the inferior cervical ganglia) make 

 the organ work harder. Changes in the action of the heart occasioned 

 reflexly by afferent (sensory) impulses from different parts of the body 

 are sometimes in one direction and sometimes in the other, depending 

 largely apparently on which of the two so-called centers receives the 

 stronger impression. This may be partly due, however, to the fact that 

 the inhibitor is much more dependent apparently than is the augmentor 

 on the length and the strength of its stimulus. The latter is sometimes 

 influenced by a stimulation so brief that it would affect the inhibitor 

 little or not at all. 



The nerves of the heart are in some sort of connection with the brain- 

 cortex as a whole. In consequence, the heart may be readily influenced 

 by impulses coming down the central nervous system from above as 

 well as by afferent impulses passing upward. Thus, occasionally one 

 meets a person who can voluntarily slow or even stop his heart-beat. 

 It is possible, indeed, that the fakirs of India who put themselves into 

 a state of artificial hibernation for long periods possess this dangerous 



