CHAP, iv.] THE VASCULAR MECHANISM. 285 



arteries. There is no adequate evidence that these vaso-dilator 

 fibres serve as channels for tonic dilating impulses or influences. 



The vaso-constrictor fibres leave the spinal cord by the anterior 

 roots of the nerves coining from the middle region only of the 

 spinal cord. In the dog, this region extends from about the first 

 or second thoracic to the fourth or fifth lumbar nerve ; and in 

 other animals is probably of corresponding extent. Leaving the 

 spinal nerves by the respective visceral branches, rami communi- 

 cantes, the fibres pass into the sympathetic system, the majority 

 joining the main sympathetic chain of ganglia in the thorax and 

 abdomen, but some, for instance those going to certain parts of 

 the intestine and some other viscera, leaving that chain on one 

 side and passing directly to more peripheral ganglia, such as the 

 solar plexus and the inferior mesenteric ganglia. From the sym- 

 pathetic chain the fibres run to their destination in such nerves 

 as the cervical sympathetic and splanchnic, those allotted to the 

 skin of the limbs and trunk running back again to join the respec- 

 tive spinal nerves. In the ganglia of the sympathetic chain or in 

 other more peripheral ganglia the fibres lose their medulla, and 

 continue their course as non-medullated fibres. 



In the intact organism the emission and distribution along 

 these vaso-constrictor fibres of tonic constrictor impulses, by which 

 general and local arterial tone is maintained and regulated, is 

 governed by a limited portion of the spinal bulb known as the 

 bulbar vaso-motor centre ; and when some change of conditions or 

 other natural stimulus brings about a change in the activity of the 

 vaso-constrictor fibres of one or more vascular areas, or of all the 

 arteries supplied with vaso-constrictor fibres, this same bulbar 

 vaso-motor centre appears in such cases to play the part of a 

 centre of reflex action. Nevertheless, in cases where the nervous 

 connections of this bulbar vaso-motor centre with a vascular area 

 are cut off by an operation, as by section of the cord, other parts 

 of the spinal cord may act as centres for the vaso-constrictor 

 fibres of the area, and possibly these subordinate centres may be 

 to a certain extent in action in the intact organism. 



The vaso-dilator fibres of whose existence we have clear and 

 undisputed experimental evidence, are very limited in distribution. 

 In the cases best known, the fibres leave certain regions of the 

 central nervous system and proceed to their destination along 

 certain cerebro-spinal nerves ; they do not lose their medulla 

 until they approach their termination. But as we have seen there 

 is evidence of vaso-dilator fibres also running in nerves of the 

 sympathetic system. The vaso-dilator fibres are generally thrown 

 into action as part of a reflex act, and the centre, in the reflex act, 

 appears in each case to lie in the central nervous system not far 

 from the origin of the ordinary motor fibres which the dilator 

 fibres accompany. 



The effects of the activity of the vaso-dilator fibres appear to be 



