CHAP, iv.] THE VASCULAR MECHANISM. 321 



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 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 con- 

 spicuous 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 blood vessels, 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 other- 

 wise may give rise to impulses passing down along the central 

 nervous system itself to the medulla oblongata, 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 medulla may, by 

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

 chorda tympani 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 lumbar or lower dorsal 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 shewn 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 brought about by stimulation of sentient 

 surfaces. And other instances might be quoted in which vaso- 

 dilator fibres appear to be connected with a 'centre' soon after 

 their entrance into the nervous system. 



If, as seems probable, 167, the blood vessels of a muscle dilate 

 by vaso-motor action whenever the muscle is thrown into con- 

 traction, either in a reflex or voluntary movement, the vaso- 

 dilator fibres of the muscle would seem to be thrown into action 

 by impulses arising in the spinal cord not far from the origin of 



F. 21 



