THE CIRCULATION OF THE BLOOD AND LYMPH 147 



mammals, on the other hand, a true antagonism seems to exist ; and 

 stimulation of the inhibitory nerves is less effective when the aug- 

 mentors are excited at the same time. The cardiac nerves affect not 

 only the rate and force of the contraction, but also the conductivity 

 of the heart. Thus in the frog's heart during stimulation of the 

 vagus, the contraction passes more slowly, and during stimulation of 

 the sympathetic more quickly from auricles to ventricle. 



In mammals (and in what follows we shall restrict ourselves chiefly 

 to the dog, cat, and rabbit, as it is in these animals that the subject 

 has been most carefully studied) the inhibitory fibres run down the 

 vagus in the neck and reach the heart 'by its cardiac branches. They 

 are derived from the bulbar roots of the spinal accessory, whose inner 



FIG. 61. FROG'S HEART. 



A, auricular, V, ventricular tracing. Ventricle beating very feebly. Vagus 

 stimulated (60 mm. between coils). Marked augmentation of ventricular beat. 



branch joins the vagus. The augmentor fibres leave the spinal cord in 

 the anterior roots of the second and third thoracic nerves, and possibly 

 to some extent by the fourth and fifth. Through the corresponding 

 white rami communicantes they reach the sympathetic cord, and run- 

 ning up through the stellate ganglion (first thoracic), and theannulus 

 of Vieussens, which surrounds the subclavian artery, to the inferior 

 cervical ganglion, they pass off to the heart by separate ' accelerator ' 

 branches, taking origin either from the annulus or from the inferior 

 cervical ganglion. Some augmentor fibres are often, if not always, 

 present in the dog's vago-sympathetic in the neck. It is especially 

 easy to demonstrate their presence five or six days after section of the 

 nerve, when the excitability of the inhibitory fibres has disappeared. 



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