1 62 



THE CIRCULATION OF THE BLOOD AND LYMPH 



GTV-- 



ductivity 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 

 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 sympa- 

 thetic cord, and running up through the 

 stellate ganglion (first thoracic), and the 

 annulus 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 demon- 

 strate their presence five or six days after 

 section of the nerve, when the excitability 

 of the inhibitory fibres has disappeared. 

 In the dog the vagus and cervical sym- 

 pathetic are, in the great majority of 

 cases, contained in a strong common 

 sheath, and pass together through the 

 inferior cervical ganglion. Upon opening 

 this sheath they may with care be separ- 

 ated, the fibres running in distinct strands, 

 and not mixed together as in the vago- 

 sympathetic of the frog. For some dis- 

 tanco below the superior cervical ganglion 

 the cervical sympathetic is not connected 

 with the vagus, and here the nerves may 

 be separately stimulated without any 

 artificial isolation. In the rabbit and some 

 other mammals, including man, the vagus 

 and sympathetic run a separate course in 

 the neck. 



Fig. 74. Diagram of Cardiac 

 Nerves in the Dog (after 

 Foster). II, III, second and 

 third dorsal nerves; SA, sub- 

 clavian artery ; AV, annulus of 

 Vieussens; ICG, inferior cer- 

 vical ganglion; CS, cervical 

 sympathetic; i, first thoracic 

 or stellate ganglion of the 

 sympathetic; 2, second thora- 

 cic ganglion; Ac., accelerator 

 or augmentor fibres passing 

 off towards the heart; X, roots 

 of vagus; XI, roots of spinal 

 accessory; JG, jugular gan- 

 glion; GTV, ganglion trunci 

 vagi; In., inhibitory fibres 

 passing off towards the heart. 



The 



effects of stimulation of the 

 vagus or vago - sympathetic in the 

 mammal are very much the same 

 as in the frog, except that secon- 

 dary augmentation is in general less marked, though often present 

 in some degree, and that in the mammal the inhibitory fibres have 

 a smaller direct action on the ventricle. It indeed beats more 

 slowly when the auricle is slowed, but this is only because in the 

 normally beating heart the ventricle takes the time from the 

 auricle. The strength of the ventricular contractions may be not at 



