CHAP, iv.] THE VASCULAR MECHANISM. 331 



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 sympa- 

 thetic, hypogastric &c., or along recurrent branches of the splanch- 

 nic 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 vaso-motor centre ; and when some change of con- 

 ditions 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 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 medullary 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-con- 

 strictor 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 im- 

 pulses, constriction, it may be of considerable 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. 



179. We shall have occasion later on again and again to point 

 out instances of the effects of vaso-motor action both local and 



