THE CIRCULATION OF THE BLOOD AND LYMPH 157 



by the inhibitory centre, which, within a considerable range, 

 can produce variations in either direction. The augmentor 

 mechanism is perhaps 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-con- 

 strictor), and of nerves which have the power of increasing it 

 (vaso- dilator). 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 convenient to distinguish the 

 afferent nerves which cause on the whole a vaso-constriction 

 and a consequent increase of arterial pressure as pressor nerves, 

 and those which cause on the whole vaso-dilatation, with fall 

 of pressure, as depressor nerves, reserving the terms vaso-con- 

 strictor and vaso-dilator for the efferent portions of the reflex 

 arcs. It is through this reflex mechanism that the bloodvessels 

 are mainly influenced, although the endings of the vaso-motor 

 nerves in the smooth muscular fibres or the muscular fibres 

 themselves are sometimes 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. 201) ; suprarenal extract, or its active principle, 

 adrenalin, constriction, with a rise of pressure (p. 201). Apoco- 

 deine paralyzes the vaso-motor nerve-endings after a preliminary 

 stimulation, and now adrenalin causes no constriction. Chryso- 

 toxin, an active principle of ergot, causes a marked rise of blood- 

 pressure by stimulating the sympathetic ganglion-cells or the 

 pre-ganglionic fibres of the vaso-constrictor path. Vaso-motor 

 nerves control chiefly the small arteries. They have no direct 

 influence on the capillaries.* Nor has the existence of an effec- 

 tive vaso-motor regulation of the calibre of the veins, except in 

 the portal system, been proved up to this time by any clear and 



* It is usually taught that the capillaries, being devoid of muscular 

 fibres in their walls, are not supplied with vaso-motor fibres, and that the 

 only kind of active contraction of which they are capable is due to a process 

 analogous to the turgescence of vegetable cells, the thickness of the wall 

 being increased at the expense of the lumen, while the total cross-section 

 of the vessel remains unchanged. It has recently been asserted, however, 

 that a true contraction, in which both the total section and the lumen are 

 diminished, may be caused in the capillaries of the nictitating membrane 

 of the frog either by direct stimulation or by excitation of vaso-motor 

 fibres in the sympathetic (Steinach and Kahn). 



