212 VASO-MOTOR CENTRES. [BOOK i, 



as mixed in character, since according to circumstances they give 

 rise either to dilation or to constriction. 



The course of vaso-motor fibres. Leaving out of consideration 

 local vaso-motor mechanisms, such as those which may be sup- 

 posed to exist in the submaxillary gland, we may make the general 

 statement that vaso-motor influences may be traced back to the 

 spinal cord. The exact paths taken by the vaso-motor fibres have 

 not however as yet been fully worked out. 



Most observers are agreed that the fibres leave the spinal cord 

 by the anterior roots of the spinal nerves ; but in the majority of 

 cases at all events as far as the mammal is concerned, the fibres do 

 not run in a direct course to their destination in company with the 

 ordinary motor fibres passing to the same structures as themselves. 

 Thus the vaso-motor fibres of the hind limbs do not pass directly 

 with the anterior roots into the sciatic nerve but, largely at all 

 events, turn aside, to join through the rami communicantes the 

 abdominal sympathetic ; and it is only after they have traversed a 

 certain length of sympathetic nerve that they again return to the 

 spinal nerves, enter into the sciatic plexus, and thus become 

 part of the nerves of the leg. So also the vaso-motor fibres for the 

 forelimb pass in large measure from the anterior roots of the upper 

 dorsal nerves to the thoracic sympathetic chain and thence by the 

 first thoracic ganglion to the brachial plexus and so on to the fore- 

 limb. And we have already seen that the vaso-motor fibres for the 

 head and face, pass from the lower cervical or lower dorsal spinal 

 cord to the first thoracic or to the last cervical ganglion and by the 

 cervical sympathetic upwards. 



When, as in the case of the submaxillary gland, the presence of 

 distinct and antagonistic vaso-constrictor and vaso-dilator nerves is 

 conspicuous in the same organ, the dilator fibres are generally found 

 running in a cerebro-spinal and the constrictor fibres in a sympa- 

 thetic nerve, but we cannot at present say that such a contrast is 

 invariable. We cannot as yet trace out such distinct courses for 

 the dilator and constrictor fibres of either the fore or hind limb ; 

 and in the tongue while dilator fibres run into the lingual nerve, 

 constrictor fibres appear in the hypoglossal which is no less clearly 

 a spinal nerve than the fifth of which the lingual is a branch. 



Vaso-motor centres. There remains the important question, 

 What part of the central nervous system is it which intermediates 

 as a nervous vaso-motor centre or centres either of purely reflex or 

 of partly reflex and partly automatic action, between various affer- 

 ent impulses and the efferent vaso-motor impulses leading either to 

 dilation or constriction ? 



We have seen that stimulation of the central stump of the 

 divided sciatic gives rise, in an animal under urari, to an increase 

 of general blood-pressure, brought about chiefly, if not entirely, by 

 an augmentation of constrictive impulses passing along the splanch- 

 nic nerves. This increase of blood-pressure is manifested, with 



