CHAP, iv.] THE VASCULAR MECHANISM. 277 



through itself, is much more marked than the general effects, the 

 change in blood pressure and the flow through other arteries. 

 When, however, the area the arteries of which are affected is large, 

 the general effects are very striking. Thus if while a tracing of 

 the blood pressure is being taken by means of a manometer 

 connected with the carotid artery, the abdominal splanchnic nerves 

 be divided, a conspicuous but steady fall of pressure is observed, 

 very similar to but more marked than that which is shewn in 

 Fig. 74. The section of the abdominal splanchnic nerves causes 

 the arteries of the abdominal viscera to dilate, and these being 

 very numerous, a large amount of peripheral resistance is taken 

 away, and the blood pressure falls accordingly ; a large increase 

 of flow into the portal veins takes place, and the supply of blood 

 to the face, arms, and legs is proportionally diminished. It will 

 be observed that the dilation of the arteries is not instantaneous 

 but somewhat gradual, as shewn by the pressure sinking not 

 abruptly but with a gentle curve. 



The general effects on blood pressure by vaso-motor changes 

 are so marked that the manometer may be used to detect vaso- 

 motor actions. Thus, if the stimulation of a particular nerve or 

 any other operation leads to a marked rise of the mean blood 

 pressure, unaccompanied by any notable changes in the heart beat, 

 we may infer that constriction has taken place in the arteries of 

 some considerable vascular area; and similarly, if the effect be 

 a fall of blood pressure, we may infer that constriction has given 

 way to dilation. 



Vaso-motor .Functions of the Central Nervous System. 



150. The central nervous system, to which we have traced 

 the vaso-motor nerves, makes use of these nerves to regulate the 

 flow of blood through the various organs and parts of the body ; 

 by the local effects thus produced it assists or otherwise influences 

 the functional activity of this or that organ or tissue ; by the 

 general effects it secures the well being of the body. When the 

 vaso-dilators are brought into play the chief effect is a local 

 one ; when a general effect has to be produced the vaso-con- 

 strictors are employed, though these of course also bring about 

 local effects. And we may consider the two separately. 



The vaso-dilator nerves, .the use of which is more simple 

 than that of the vaso-constrictors in so far as it appears not 

 to be complicated by the presence of habitual tonic influences, 

 occur as parts of distinct mechanisms used chiefly at least as 

 reflex mechanisms, with centres placed in different regions of the 

 central nervous system. Thus, when food is placed in the mouth 

 afferent impulses, generated in the nerves of taste, give rise in 

 the central nervous system to efferent impulses, which descend 



