CHAP, iv.] THE VASCULAR MECHANISM. 351 



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 

 sympathetic 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 

 respective 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 con- 

 nections 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 

 essentially local in nature. When any set of the fibres come into 

 action the vascular area which these govern is dilated ; and the 

 vascular areas so governed are relatively so small that changes in 

 them produce little or no effect on the vascular system in general ; 

 the fibres are called into play to produce special effects in special 

 organs. 



The effects of changes in the activity of the vaso-constrictor 

 fibres are both local and general. They are also double in 

 nature ; by an inhibition of tonic constrictor impulses a certain 

 amount of dilation may be effected ; by an augmentation of 

 constrictor impulses, constriction, it may be of considerable extent, 



