340 USE OF VASODILATOR FIBRES. [BOOK i. 



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. 



172. 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 

 the chorda tympani and other nerves to the salivary glands and, 

 by dilating the blood vessels, secure a copious flow of blood 

 through the glands while, as we shall see later on, they excite 

 the glands to secrete. The centre of this reflex action appears 

 to lie in the spinal bulb and may be thrown into activity not 

 only by impulses reaching it along the specific nerves of taste, 

 but also by impulses passing along other channels ; thus, emotions 

 started in the brain by the sight of food or otherwise may give 

 rise to impulses passing down along the central nervous system 

 itself to the spinal bulb, or events in the stomach may send 

 impulses up the vagus nerve, or stimulation of one kind or another 

 may send impulses up almost any sentient nerve, and these 

 various impulses reaching the spinal bulb may, by reflex action, 

 throw into activity the vaso-dilator fibres of the chorda tympani 



