THE CIRCULATION OF THE BLOOD. 247 



cerebrum may cause vase-dilatation as in blushing, or vase-constriction 

 as in the pallor of fear. An important reflex association exists between 

 the vessels of the skin and those of subjacent parts. It is generally an 

 inverse relation; that is, when the superficial vessels are dilated the 

 deep are contracted. This reflex is made use of in medical practice 

 when poultices are applied to the chest in pneumonia, the lung being in 

 a state of inflammation. 



Afferent influence upon the vaso-motor centre is well shown by the 

 action of a nerve called the depressor, the existence of which was de- 

 monstrated by Cyon and Ludwig. 



Fig. 199. Tracing showing the effect on blood-pressure of stimulating the central end of the 

 Depressor nerve in the rabbit. To be read from right to left. T, indicates the rate at which the 

 recording surface was travelling, the intervals correspond to seconds; C. the moment of entrance of 

 current; O, moment at which it was shut off. The effect is some time in developing and lasts afr^r 

 the current has been taken off. The larger undulations are the respiratory nerves; the pulse oscilla- 

 tions are very small. (Foster.) 



It is a small afferent nerve and passes up from the heart in which it 

 takes its origin, and in the rabbit goes upward in the sheath of the su- 

 perior laryngeal branch of the vagus or with that branch and the vagus 

 itself, communicating by filaments with the inferior cervical ganglion 

 as it proceeds from the heart. 



If during a blood-pressure observation in a rabbit this nerve be di- 

 vided, and the central end (i.e., that nearest the brain) be stimulated, a 

 remarkable fall of blood-pressure takes place (fig. 199). 



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 the vessels hold a much 

 larger quantity of blood than usual. The engorgement of the splanch- 

 nic area very greatly diminishes the amount of blood in the vessels else- 

 where, and so materially diminishes the blood-pressure. The function 

 of the depressor nerve is that of conveying to the vaso-motor centre in- 

 dications of such conditions of the heart as require a diminution of the 



