VA80-DILATOR NERVES. 255 



the other hand, certain fibres conveying impulses from the 

 heart inhibit the centre and dilate the arteries^ Jlpwer 

 blood-pressure, and diminish the resistance to be overcome 

 "hy frfrfi frfiart. These fibres run in branches of the pneumo- 

 gastric, and are known as the depressor fibres, or in certain 

 animals, for example the rabbit, where they are all collected 

 into one branch, as the depressor nerve. If this nerve be 

 divided and its cardiac end stimulated no effect is pro- 

 duced, but if its central end (that still connected 

 with the rest of the pneumogastric trunk and through it 

 with the medulla oblongata) be stimulated, arterial pressure 

 gradually falls; this result being dependent upon a dilata- 

 tion of the small arteries, and consequent diminution of 

 the peripheral resistance, following an inhibition of the 

 vaso-motor centre brought about by the depressor nerve. 

 Through the depressor nerve the heart can therefore influ- 

 ence the calibre of the small arteries and, by lowering 

 aortic pressure, diminish its own work if need be. 



Blushing. The depressor nerves control a great part of 

 the vaso-motor centre, and so can bring about dilatation 

 of a large number of arteries their influence is called into 

 play when general arterial pressure is to be lowered, but is 

 useless for controlling local blood-supply. This is man- 

 .aged by other afferent nerves, each of which inhibits a 

 small part only of the vaso-motor centre, governing the 

 arteries of a limited tract of the Body; the dilatation of 

 these increases the amount of blood flowing through the 

 particular region to which they are distributed, but does 

 not affect the total resistance to the blood-flow sufficiently 

 to influence noticeably the general pressure in the arterial 

 system. In blushing, for example, under the influence of 

 an emotion, that part of the vaso-motor centre which sup- 

 plies constrictor nerves to the arteries of the skin of the 

 neck and face, is inhibited by nerve-fibres proceeding from 

 the cerebrum to the medulla oblongata, and the face and 

 neck consequently become full of blood and flush up. 

 Quite similar phenomena occur under other conditions in 

 many parts of the Body, although when not visible on the 

 surface we do not usually call them blushes. The mucous 



