CHAP, iv.] THE VASCULAR MECHANISM. 203 



tone, which indeed might be at once and for ever arranged for by 

 the proper natural calibre of the elastic blood-vessels, is the power 

 which the central nervous system possesses of varying the tone of 

 this or that artery or group of arteries, of increasing it or of 

 diminishing it, of producing constriction or dilation in those arteries, 

 and thus, as we have seen, of effecting changes in general or 

 local blood-pressure or in both, and consequently of determining a 

 flow of blood in this or that direction, according to the needs of the 

 economy. And the exercise of this carefully arranged manipulation 

 of the muscular walls of the arteries may be called forth in either 

 direction, in the way of constriction, or in the way of dilation (or 

 of both at the same time, one in one area and the other in others), 

 by means of nervous impulses either originating in the central 

 nervous system itself or started by afferent impulses passing up to 

 the central nervous system from some sentient surface. 



Blushing is a familiar instance of vascular dilation brought 

 about by the action of the central nervous system. Nervous im- 

 pulses started in some parts of the brain by an emotion produce 

 certain changes in the central nervous system (the exact nature 

 and locality of these changes we shall discuss presently) which 

 have in turn an effect on the vaso-motor fibres of the cervical 

 sympathetic almost exactly the same as that produced by section 

 of the nerve. In consequence the muscular walls of the arteries of 

 the head and face relax, the arteries dilate and the whole region 

 becomes suffused. Sometimes an emotion gives rise not to blushing, 

 but to the opposite, viz. to pallor. In a great number of cases this 

 has quite a different cause, being due to a sudden diminution or 

 even temporary arrest of the heart's beats; but in some cases 

 it may occur without any change in the beat of the heart, and is 

 then due to a condition the very converse of that of blushing, that 

 is, to an increased arterial constriction; and this increased con- 

 striction, like the dilation of blushing, is effected through the 

 agency of the central nervous system and the cervical sympathetic. 

 These are familiar examples, but we have in abundance exact 

 experimental evidence of the effect of afferent impulses in inducing 

 through the central nervous system vaso-motor changes and thus 

 bringing about sometimes constriction, sometimes dilation, some- 

 times the two together. The action of the so-called depressor 

 nerve is a striking instance of reflex dilation as it may be called. 



If in the rabbit while the pressure in an artery such as the 

 carotid is being registered, the depressor nerve, which is a branch 

 of the vagus running alongside the carotid artery and sympathetic 

 nerve (Fig. 41, n. dep.), be divided, and its central end (i. e. the one 

 connected with the brain) be stimulated with the interrupted 

 current, a gradual but marked fall of pressure in the carotid 

 is observed, lasting, where the period of stimulation is short, 

 some time after the removal of the stimulus (Fig. 46). Since the 

 beat of the heart is not markedly changed, the fall of pressure 



