THE NERVOUS REGULATION OF THE BLOODVESSELS 171 



SECTION VI. THE NERVOUS REGULATION OF THE BLOODVESSELS 

 (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 theft 

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

 increasing it (vaso-dilator). All nerves that effect 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-constrictor 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 sub- 

 stances circulating in the blood. Proteoses, for instance, cause by 

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

 (p. 213); suprarenal extract, or its active principle, adrenalin, or 

 epinephrin, constriction, with a rise of pressure (pp. 214, 638). Apo- 

 codeine 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 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 unambiguous experi- 

 ment, 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. 



* 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 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) . 



