I4 -4 MANUAL OF PHYSIOLOGY 



merely auxiliary to the inhibitory, being called into action 

 only in emergencies. 



Vaso-motor Nerves. Just as the muscular walls of the heart 

 are governed by two sets of nerve-fibres, a set which keeps 

 down the rate of working and a set which may increase it, 

 the muscular walls of the vessels are under the control of 

 nerves which have the power of diminishing their calibre 

 (vaso-constrictor), and of nerves which have the power of 

 increasing it (vasodilator). All nerves that affect the calibre 

 of the vessels, whether vaso-constrictor or vaso-dilator, are 

 included under the general name vaso-motor. These vaso- 

 motor nerves, like the augmentor and inhibitory fibres of 

 the heart, are connected with a centre or centres, which in 

 turn are in relation with numerous afferent nerves. It is 

 through this reflex mechanism that the blood-vessels are 

 mainly influenced, although the endings of the vaso-motor 

 nerves in the smooth muscular fibres or the muscular fibres 

 themselves may sometimes be directly affected by substances 

 circulating in the blood. Albumose, for instance, causes by 

 peripheral action dilatation of the vessels and a fall of blood- 

 pressure (p. 188), suprarenal extract constriction, with a rise 

 of pressure (p. 189). Vaso-motor nerves control chiefly the 

 small arteries. They have no direct influence on the capil- 

 laries. Nor has the existence of an effective vaso-motor 

 regulation of the calibre of the veins, except in the portal 

 system, been proved up to this time by any clear and un- 

 ambiguous experiment, although there are grounds on which 

 it has been surmised that the nervous system does influence 

 the ' tone ' of the whole venous tract. These grounds will 

 be mentioned in the proper place. Meanwhile, before de- 

 scribing the distribution of the best-known tracts of vaso- 

 motor fibres and defining the position of the vaso-motor 

 centres, we must glance at the methods by which our know- 

 ledge has been attained. 



(i) In translucent parts inspection is sufficient. Paling of the part 

 indicates constriction ; flushing, dilatation of the small vessels. This 

 method has been much used, sometimes in conjunction with (2) in 

 such parts as the balls of the toes of dogs or cats, the ear of the 

 rabbit, the conjunctiva, the mucous membrane of the mouth and 

 gums, the web of the frog, the wing of the bat, the intestines, etc. 



