154 HAND-BOOK OF PHYSIOLOGY. 



body there exists a nerve the division of which produces the same effects, 

 viz., dilatation of the arteries; such may be cited as the case with the 

 sciatic, the splanchnic nerves, and the nerves of the brachial plexus: 

 when divided, dilatation of the blood-vessels in the parts supplied by 

 them taking place. It appears, therefore, that nerves exist which have a 

 distinct control over the vascular supply of a part. 



These nerves are called vaso-motor; or, since they seem to run now 

 in cerebro-spinal nerves, now in the sympathetic, we speak of those 

 nerves as containing vaso-motor fibres, in addition to the fibres which 

 have other functions. 



Vaso-motor centres. Experiments by Ludwig and others show 

 that the vaso-motor fibres come primarily from grey matter (vaso-motor 

 centre) in the interior of the medulla oblongata, between the calamus 

 scriptorius and the corpora quadrigemina. Thence the vaso-motor fibres 

 pass down in the interior of the spinal cord, and issuing with the anterior 

 roots of the spinal nerves, traverse the various ganglia on the pras-vertebral 

 cord of the sympathetic, and, accompanied by branches from these 

 ganglia, pass to their destination. 



Secondary or subordinate centres exist in the spinal cord, and local 

 centres in various regions of the body, and through these, directly under 

 ordinary circumstances, vaso-motor changes are also effected. 



The influence exerted by the chief vaso-motor centre is called into 

 play in several ways, but chiefly by afferent (sensory) stimuli, and it may 

 be exerted in two ways, either to increase its usual action which main- 

 tains a medium tone of the arteries or to diminish such action. This 

 afferent influence upon the centre may be extremely well shown by the 

 action of a nerve the existence of which was demonstrated by Cyon and 

 Ludwig, and which is called the depressor, because of its characteristic 

 influence on the blood-pressure. 



Depressor Nerve. This small nerve arises, in the rabbit, from the 

 superior laryngeal branch, or from this and the trunk of the pneumogas- 

 tric nerve, and after communicating with filaments of the inferior cervical 

 ganglion proceeds to the heart. 



If during an observation of the blood-pressure of a rabbit this nerve 

 be divided, and the central end (i.e., that nearest the brain) be stimu- 

 lated, a remarkable fall of blood-pressure ensues (Fig. 139). 



The cause of the fall of blood-pressure is found to proceed from the 

 dilatation of the vascular district supplied by the splanchnic nerves, in 

 consequence of which it holds a much larger quantity of blood than usual, 

 and this very greatly diminishes the blood in the vessels elsewhere, and 

 so materially affects the blood-pressure. This effect of the depressor nerve 

 is presumed to prove that the nerve is a means of conveying to the vaso- 

 motor centre indications of such conditions of the heart as require a 

 diminution of the tension in the blood-vessels; as, for example, when the 



