THE ACCELERATOR NERVES 



and run down to the heart within a common sheath, forming the vago- 

 sympathetic trunk. Stimulation of the accelerators of the frog must be 

 applied to the pathway before the fibers join the common trunk if uncom- 

 plicated augmentation is to be secured. On stimulation of the mixed 

 vago-sympathetic trunk inhibition ordinarily occurs at once. Augmentor 

 effects come on only after the inhibition has disappeared, usually fifteen 

 or twenty seconds later. If the vagal influence is first removed by a 

 specific poison, atropine, then on stimulation pure augmentation results 

 at once. This method applied to the frog is one of the most satisfactory 

 methods of illustrating the different elements in cardiac augmentation. 



In the dog the augmentor fibers leave the cord by the anterior roots of 

 the second and third dorsal nerves, and possibly also by the first, fourth, 

 and fifth dorsal nerves. They pass by the rami communicantes to the gang- 

 lion stellatum, or first thoracic ganglion, and around the ansa to the inferior 

 cervical ganglion of the sympathetic. Fibers from the ansa or from the 

 inferior cervical ganglion proceed to the heart, figure 183. The course of 

 the augmentor fibers in the spinal cord is not so well known except that 

 they originate in an augmentor center in the medulla. The circulation 

 of venous blood appears to stimulate the augmentor center, and of highly 

 oxygenated blood the inhibitory center. 



The accelerator center, like the inhibitory, is in constant tonic activity; 

 and the cardiac acceleration on cutting the vagi, shown in figure 182, is in 

 part to be ascribed to this tone. When both nerves are stimulated 

 together, the resulting rate is the algebraic sum of the opposed influences, 

 according to Hunt. The accelerator center is influenced by afferent 

 impulses arising throughout the body, and these reflexes contribute to the 

 general co-ordination of the chest with the activities of the body. 



In addition to direct and reflex stimulation, impulses passing down from 

 the cerebrum may have a similar effect, psychic stimulation. 



Other Influences which Affect the Heart. A great variety of special 

 conditions influence the heart's action in the normal body, conditions that 

 are not discussed directly under any of the categories treated above. Of 

 these may be mentioned the coronary circulation, temperature, mechanical 

 tension, age. 



The Coronary Circulation. The contractions of the heart cannot long 

 be maintained without a due supply of blood or other nutrient fluid. The 

 nutrient fluid for the heart of man and the mammals is supplied from the 

 coronary arteries and the vessels of Thebesius. The coronary arteries arise 

 from the base of the aorta, where they receive the benefit of the highest 

 arterial pressure. The coronary arteries are terminal arteries; that is, 

 they do not permit the establishment of a collateral circulation when one 

 of their branches is blocked. If the block be complete, that portion of the 

 heart wall supplied by the branch dies. The immediate effect of the 



