202 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



The vascular reflex ' may appear in a part associated in function with the 

 sensory surface stimulated. Thus the stimulation of the tongue causes dilata- 

 tion of the blood-vessels in the submaxillary gland. Frequently the vascular 

 reflex is seen on both ^i«lts of the body. The stimulation of the mucous 

 membrane on one side of the nose may cause vascular dilatation in the whole 

 head; the effect in this case is usually more marked on the side stimulated. 

 The vessels of one hand contract when the other hand is put in cold water. 

 Sometimes distant and apparently unrelated parts are affected. Vulpian 

 noticed that the stimulation of the central end of the sciatic caused the vessels 

 of the tongue to contract. 2 



The vascular changes produced reflexly in the splanchnic area are of 

 especial importance because of the great number of vessels innervated through 

 these nerves and the great changes in the blood-pressure that can follow dilata- 

 tion or constriction on so large a scale. 



There is in some degree an inverse relation between the vessels of the skin 

 and deeper parts on reflex stimulation of the vaso-motor centres. The super- 

 ficial vessels are often dilated while those of deeper parts are constricted. 3 

 Thus the stimulation of the central end of the sciatic nerve may cause a dilata- 

 tion of the vessels of the lips, hand in hand with a rise in general blood-pres- 

 sure. 4 Exposing a loop of intestine dilates the intestinal vessels in the rabbit, 

 but constricts those of the ear. In asphyxia, the superficial vessels of the ear, 

 face, and extremities dilate, while the vessels of the intestine, spleen, kidneys 

 and uterus are constricted. 



Relation of Cerebrum to Vaso-motor Centres. — A rise of general blood- 

 pressure has been produced by the stimulation of different regions of the cortex 

 and of various other parts of the brain ; for example, the crura cerebri and 

 corpora quadrigemina. Yaso-dilatation has also been observed. The motor 

 area of the cortex especially seems closely connected with the bulbar vaso- 

 motor centres. There is, however, no conclusive evidence that special vaso- 

 motor centres exist in the brain aside from the bulbar centres already described. 

 At present the safer view is that the changes in blood-pressure called forth by 

 the stimulation of various parts of the brain are reflex actions, the afferent im- 

 pulse starting in the brain as it might in any other tissue peripheral to the 

 vaso-motor centres. 



Pressor and Depressor Fibres. — The stimulation of the same afferent 

 nerve sometimes causes reflex dilation of the vessels of a part, instead of the 

 more usual reflex constriction. Two explanations of this fact have been sug- 

 gested. The first assumes that the condition of the vaso-motor centre varies in 

 such a way that the same stimuli might produce contrary effects, depending 

 on the relation between the time of stimulation and the condition of the centre. 



1 The general arrangement of the matter in this paragraph is that given hy Tigerstedt, Der 



mf 1893, p. 519. 

 * Compare Sergejew : Gentralblatt fur die medicinische Wissemehaft, 1894, ]>. 162. 

 s Wertheimer : Gomptes rendus, 1893, cxvi. p. 595; rlallion and Franck: Archives de Physi- 

 olo'ii , 1896, p. 502; Bayliss and Bradford: Journal of Physiology, 1SU4, xvi. p. 17. 

 i [sergin: Arehivfur Physiologic, L894, p. 448. 



