230 THE VASCULAR MECHANISM. 



and probably in other mammals from about the first dorsal to the fifth 

 lumbar nerve), pass into the splanchnic ganglia connected with those nerves 

 (thoracic and abdominal chain of sympathetic ganglia), where the fibres lose 

 their medulla, and proceed to their destination as non-medullated fibres, 

 either still in so-called sympathetic nerves, such as splanchnic, cervical 

 sympathetic, hypogastric, etc., or along recurrent branches of the splanchnic 

 system, to join the spinal nerves of the arm, leg, and trunk. 



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 medulla oblongata known as the medullary vasomotor 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 

 medullary vasomotor centre appears in such cases to play the part of a 

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

 of this medullary vasomotor 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 appear to take origin in various parts of the 

 central nervous system and to proceed in a direct course to their destination 

 along the (anterior) roots and as part of the trunks and branches of various 

 cerebro-spinal nerves ; they do not lose their medulla until they approach 

 their termination. They do not appear to serve as channels of tonic dilating 

 influences ; they are thrown into action generally as part of a reflex action, 

 and their centre in the reflex act appears in each case to lie in the central 

 nervous system not far from the centre of the ordinary motor fibres which 

 they accompany. 



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

 local in nature ; when any set of them 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 effects of changes in the activity of the vaso-constrictor fibres are 

 both local and general, and may be 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 con- 

 siderable extent, may be brought about. When the vascular area so affected 

 is small, the effects are local, more or less blood is distributed through the 

 area ; when the vascular area affected is large, the inhibition of constriction 

 may lead to a marked fall, and an augmentation of constriction to a marked 

 rise of general blood-pressure. 



165. We shall have occasion later on again and again to point out 

 instances of the effects of vasomotor action, both local and general, but we 

 may here quote one or two characteristic ones. " Blushing " is one. Ner- 

 vous impulses started in some parts of the brain by an emotion produce a 

 powerful inhibition of that part of the medullary vasomotor centre which 

 governs the vascular areas of the head supplied by the cervical sympathetic, 

 and hence has an effect on the vasomotor fibres of the cervical sympathetic 

 almost exactly the same as that produced by section of the nerve. In conse- 

 quence the muscular walls of the arteries of the head and face relax, the 

 arteries dilate, and the whole region becomes suffused. Sometimes an emo- 

 tion gives rise not to blushing, but to the opposite effect, viz., to pallor. 



