THE ACCELERATOR NERVES 183 



powerful than is the case with the vagus stimulation; second, a longer time 

 elapses before the effect is manifest; and third, the augmentation is followed 

 by exhaustion, the beats being after a time feeble and less frequent. The 

 stimulation of the vago-sympathetic in the frog, which usually produces 

 inhibition, will occasionally produce acceleration, especially if the heart is 

 beating feebly at the time of the stimulation. 



The fibers of the sympathetic system, which influence the heart-beat in 

 the frog, leave the spinal cord by the anterior root of the third spinal nerve. 

 They pass by the ramus communicans to the third sympathetic ganglion, 

 thence to the second ganglion, the annulus of Vieussens (around the 

 subclavian artery), through the first ganglion, and along the main trunk 

 of the sympathetic to near the exit of the vagus from the cranium. There 

 the two nerves join and run down to the heart within a common sheath, 

 forming the vago-sympathetic trunk. 



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 gan- 

 glion stellatum, or first thoracic ganglion, around the annulus of Vieussens 

 to the inferior cervical ganglion of the sympathetic. Fibers from the annulus 

 or from the inferior cervical ganglion proceed to the heart, figure 181. 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 180, 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 coordina- 

 tion 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. 



Other Influences Which Affect the Heart. A great variety of spe- 

 cial 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, sex, etc. 



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 



