THE CIRCULATION OF THE BLOOD. 149 



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 these are divided, dilatation of the blood-vessels in the 

 parts supplied by them takes place. It appears, therefore, that nerves 

 exist which have a distinct control over the vascular supply of every part 

 of the body. 



These nerves are called vaso-motor ; they run now in cerebro-spinal, 

 now in the sympathetic nerve-trunks. 



Vaso-motor centres. Experiments by Ludwig and others show that 

 the vaso-motor fibres come primarily from gray 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 prae-vertebral cord 

 of the sympathetic, and, accompanied by branches from those ganglia', 

 pass to their distination. 



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 not only in 

 constant moderate action, but maybe altered in several ways, but chiefly 

 by afferent (sensory) stimuli. These stimuli may act in two ways, either 

 increasing or diminishing the usual action of the centre, which maintains 

 a medium tone of the arteries. This afferent influence upon the centre 

 may be extremely well shown by the action of a nerve the existence of 

 which was demonstrated by Oyon 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 cer- 

 vical 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. 138). 



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

 dilatation of the vascular district within the abdomen supplied by the 

 splanchnic nerves, in consequence of which it holds a much larger quan- 

 tity of blood than usual. The engorgement of the splanchnic area very 

 greatly diminishes the blood in the vessels elsewhere, and so materially 

 diminishes the blood-pressure. The function of the depressor nerve is 

 presumed to be that 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, that the heart cannot, with sufficient 

 ease, propel blood into the already too full or too tense arteries. 



