CHAP, iv.] THE VASCULAR MECHANISM. 299 



r. Vfj. roots of the vagus ; r. Sp. Ac. roots of the spinal accessory : both drawn 

 very cliagrammatically. G. J. ganglion jugulare. G. Tr. Vg. ganglion truiici vagi. 

 Sp. Ac. spinal accessory trunk. Ext. Sp. Ac. external spinal accessory. i.Sp.Ac. 

 internal spinal accessory. V<j. trunk of vagus nerve, n. c. branches going to 

 heart. C. Sy. cervical sympathetic. G. C. lower cervical ganglion. A. sb. sub- 

 clavian artery. .4?;. T'. Annulus of Vieussens. G.St. (Th. 1 ) ganglion stellatum 

 or 1st thoracic ganglion. G. T/f, 2 , G. Th. 3 , G. Th.*, second, third and fourth 

 thoracic ganglia. D. n., Z). in., D. iv., D. v., second, third, fourth and fifth thoracic 

 spinal nerves, r. c. ramus communicans. n. c. nerves (cardiac) passing to heart 

 (superior vena cava) from cervical ganglion and from the annulus of Vieussens. 



The inhibitory fibres, shewn by black line, run in the upper (medullary) roots of 

 the spinal accessory, by the internal branch of the spinal accessory, past the 

 ganglion trunci vagi, along the trunk of the vagus, and so by branches to the 

 superior vena cava and the heart. 



The augmentor fibres, also shewn by black line, pass from the spinal cord by the 

 anterior roots of the second and third thoracic nerves (possibly also from fourth 

 and fifth as indicated by broken black line), pass the second and first (stellate) 

 thoracic ganglia by the annulus of Vieussens to the lower cervical ganglion, from 

 whence, as also from the annulus itself, they pass along the cardiac nerves to the 

 superior vena cava. 



cervical ganglion. Their further course to the heart is along the 

 nerves springing either from the inferior cervical ganglion or from 

 the loop of "Vieussens directly. Their exact path from the ganglia 

 in fact seems to vary in different individuals. 



The path of the augmentor fibres has not been worked out so 

 fully in other mammals as in the dog, but it is most probable that 

 in all cases they leave the spinal cord by the anterior roots of the 

 second and third dorsal nerves (possibly also by the fourth and 

 fifth) and, passing up the sympathetic chain to the ganglion 

 stellatum and annulus of Vieussens, proceed to the heart by 

 nerves branching off from some part or other of the annulus or 

 from the lower and middle cervical ganglia. 



The effects of stimulating these augmentor fibres in the 

 mammal are, in general, the same as those witnessed in the frog. 

 In the mammal as in the frog impulses along these augmentor 

 fibres may be originated in the central nervous system, and 

 that probably in various ways. That palpitation of the heart 

 which is so conspicuous an effect of certain emotions is probably 

 due to the sudden positive action of augmenting impulses, though 

 it may possibly be due in part at least to sudden withdrawal of 

 normal, continuous, tonic, inhibitory impulses. 



In the mammal then as in the frog, the heart is governed by 

 two sets of nerves, the one antagonistic to the other. In the dog 

 the roots of the spinal accessory nerve, by which inhibitory fibres 

 leave the central nervous system, consist entirely of medullated 

 fibres. Among these are fibres of fine calibre, 2/A 3/x in diameter, 

 which may be traced down the trunk of the vagus, along the 

 branches going to the heart, right down to the heart itself. There 

 can be little doubt that these medullated fibres of fine calibre are 

 the inhibitory fibres of the vagus, and indeedVthere is evidence^ 

 which renders it probable that the inhibitory fibres of the heart 



