Chap, xiii.] CIECPLATION. 215 



dilatation of the capillaiies. Physical, chemical, mechanical 

 irritants can redden the skin, that is to say, cause the dilatation 

 of the capillaries. The application of a refrigerant mixture pro- 

 votes first of all the dilatation of the capillaries, then -their con- 

 tractiori. The skin becomes pale and exsanguine. On the 

 contrary, heat dilates the capillaries and congests them. 



Bright light acts as heat acts. An electric light produced by 

 120 Bunsen elements has been seen to cause a cutaneous ery- 

 thema. From diverse experiments made by M. Bouchard with 

 prisms and lenses, it results that it is the chemical rays of the 

 luminous spectrum which produce redness and vesication soonest. 

 The dilatation of the capillaries can be provoked by direct 

 irritation without the intervention of the nervous system ; for in 

 the embryon of a chicken still destitute of nerves we need only 

 deposit a drop of nicotine on the arta vasculosa to produce a 

 very beautiful congestion.' 



This fact seems to demonstrate that the capillaries, though 

 lacking njuscular fibro cells, have a contraction peculiarly their 

 own. At all events, the capillaries with simple homogeneous 

 wall dep.nd on larger capillaries, furnished with fibro-cells, and 

 especially on the arterioles and arteries, which are most evidently 

 contractile. It is therefore very natural that in acting on the 

 nervous system we can cause either the contraction or the dila- 

 tation of the capillaries. A sharp irritation of the skin, or a 

 violent pain, brings on the contraction of nearly all the capillaries.^ 

 ]f we excite in a dog the central end of the sciatic nerve sec- 

 tionised, we see contracting first the vessels of the other member, 

 then those of the whole body. 



On applying ice to the cubital nerve Waller saw the little 

 finger and the fourth finger grow red, and their temperature 

 rise. The sanguineous vessels depend, then, in a large measure 

 on the nervous system. Modern physiology has put beyond 

 doubt this interesting fact, and it has demonstrated the existence 

 • Vulpian, Lefons swr VAppa/reil Vaso-motev/r, t. I., p. 171. 

 " Ibid. p. 217. 



