760 THE CARDIAC AUGMENTOR CENTER. 



After removal of the irritation, the heart still remains for a short time in a 

 state of rest. Irritation of the vagus has thus an inhibitory after-effect. 4. Also 

 chemical irritation of the center is effective: a crystal of sodium chlorid placed 

 upon the medulla of the frog inhibits the heart-beat. 5. If the heart has 

 been arrested by irritation of the vagus, it makes a single coordinated contraction 

 on direct irritation (for example by a needle-prick) , although the contractions of 

 the heart in vagus-arrest, both after irritation, as well as those that arise secondarily 

 in a portion of the heart in consequence of irritation of another portion, take 

 place with greater difficulty, especially in the auricles 6. In the water-turtle 

 the inhibitory fibers are, according to A. B. Mayer, contained only in the right 

 vagus. Landois found this by no means constant in rabbits. 7. The nerve can 

 occasionally be successfully stimulated mechanically also in human beings by 

 digital compression against the cervical portion of the vertebral column ; although 

 alarming attacks of syncope have been observed to follow^ this procedure and for 

 this reason its practice is to be cautioned against. 8. The behavior of the vagus 

 ne.rve in the electrotonic state has been considered on p. 663 and the contraction- 

 law for the same nerve on p. 665. 9. Schiff found that irritation of the vagus 

 in the frog caused acceleration of the pulse (through an action upon the accelerator 

 fibers contained in the vagus), after he had displaced the blood in the heart by 

 a solution of sodium chlorid. If, subsequently, blood-serum is again introduced 

 into the heart, the inhibitory action of the vagus is restored. 10. Many sodium- 

 salts, naturally in suitable concentration, are capable of abolishing the inhibitory 

 action of the vagi; while, conversely, potassium-salts possess the property of 

 restoring the inhibitory function of the vagi suspended by the action of sodium- 

 salts. Both sodium-salts and potassium-salts, however, can, after protracted 

 action, induce a state in which the restitution of the suspended inhibitory function 

 of the vagi is no longer possible. Under such circumstances the heart-beat is 

 generally arrhythmic, n. If the pulsations of the heart are greatly accelerated 

 in consequence of high intracardiac pressure the activity of the cardiac branches 

 of the vagus is correspondingly diminished. This is the case also in connection 

 with the simultaneous action of direct cardiac irritants. The lessened activity of 

 the vagus in conjunction with high internal pressure within the heart occurs only 

 if the auricles and the venous sinus (in the frog) are at the same time greatly 

 distended. 12. In the frog reduction in temperature impairs the inhibitory in- 

 fluence of the vagus, while elevation of temperature increases it. In the newborn 

 and in the state of hibernation the irritation is ineffective. 



Among poisons muscarin irritates the terminations of the vagus in the heart, 

 and it may even cause diastolic arrest, which may then be neutralized by atropin. 

 Digitalin reduces the frequency of the heart-beat by irritation of the vagus-center. 

 Doses of considerable size diminish the irritability of the vagus-center and at the 

 same time increase that of the accelerator ganglia of the heart, so that the fre- 

 quency of the heart-beat is increased. In small doses digitalin also increases the 

 blood-pressure by irritation of the vasomotor center and the structures of the 

 vessel-walls. Nicotin first stimulates the vagus (and the resulting arrest can be 

 neutralized by curare or atropin) and then paralyzes it; as does also hydrocyanic 

 acid. Atropin and curare paralyze the vagi, as do marked reduction in tempera- 

 ture and high fever. 



THE CENTER FOR THE ACCELERATOR AND AUGMENTING 



CARDIAC NERVES AND THE FIBERS TO WHICH IT 



GIVES RISE. 



It is more than probable that the medulla oblongata contains a center 

 that, on the one hand, sends to the heart accelerating fibers, increasing 

 the number of heart -beats, and, on the other hand, fibers that increase 

 its systolic force. These pass from the medulla, in which the exact situa- 

 tion of their origin has not yet been determined, downward in the spinal 

 cord and enter through the communicating branches of the inferior cervi- 

 cal and the six upper thoracic nerves into the sympathetic. Thence, a 

 main branch of these fibers passes principally through the first thoracic 

 ganglion of the sympathetic and the loop of Vieussens, and hence to the 

 cardiac plexus. This nerve is designated the accelerator nerve of the 



