222 THE VASCULAR MECHANISM. 



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 abdominanl splanchnic nerves be divided, a conspicuous but steady fall 

 of pressure is observed, very similar to but more marked than that which is 

 seen in Fig. 99. The section of the abdominal splanchnic nerves causes 

 the mesenteric and other abdominal arteries 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 pro- 

 portionately diminished. It will be observed that the dilatation of the arteries 

 is not instantaneous but somewhat gradual, as shown by the pressure sinking 

 not abruptly but with a gentle curve. 



The general effects on blood-pressure by vasomotor changes are so 

 marked that the manometer may be used to detect vasomotor 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 

 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 

 dilatation. 



Vasomotor Functions of the Central Nervous System. 



158. 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 tissue; 

 by the general effects it secures the well-being of the body. 



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

 vaso-constrictors since it appears not to be complicated by the presence of 

 habitual tonic influences, is frequently conspicuous as part of a reflex act. 

 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 bloodvessels, secure a copious flow of blood through the 

 glands, while, as we shall see later on, they excite them to secrete. The 

 centre of this reflex action appears to lie in the medulla oblongata, 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 

 medulla oblongata, or even 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 medulla may, by 

 reflex action, throw into activity the vaso-dilator fibres of the chorda tym- 

 pani and other analogous nerves, and bring about a flushing of the salivary 

 glands, while at the same time they cause the glands to secrete. 



The vaso-dilator fibres of the nervi erigentes may be thrown into activity 

 in a similar reflex way, the centre in this case being placed in the sacral or 

 lumbar portion of the spinal cord, though it is easily thrown into activity 

 by impulses descending down the spinal cord from the brain that such a 

 centre does exist, is shown by the fact that when in a dog the spinal cord is 

 completely divided in the dorsal region, erection of the penis may readily be 



