DILATATION OF ARTERIES IN MUSCLES. 405 



this to take place is when the arterioles are greatlj' dilated, and 

 the blood Hoavs so quickly through them that there is no time 

 for it to be deprived of oxygen during its passage. This is seen 

 in the submaxillary gland during irritation of the chorda 

 tympani nerve, and it was observed by Meyer,* the celebrat'id pro- 

 pounder of the doctrine of conservation of energy, in persons 

 whom he bled in the tropics, and who had their vessels dilated 

 in consequence of the heat ; and it was also noticed by Crawfordf 

 in animals bled during immersion in a warm bath. It would 

 therefore seem that in fainting the vessels of the external parts 

 of the body are occasionally, at least, widely dilated, and this 

 explains the frequency witli which persons faint in warm rooms 

 and crowded churches. I am inclined then to suppose, that in 

 fainting there is dilatation of the vessels in the external parts of 

 the body, although the data on which I found my opinion are 

 too imperfect to allow of my speaking very positively on the 

 subject. If you examine the veins on the back of your hand 

 in a crowded assembly, such as people often faint in, you will 

 probably find them very full, indicating that blood is flowing 

 rapidly into them from the arteries, and that their colour 

 is of a lighter blue than usual, showing that the blood 

 they contain is lighter coloured or less venous than usual. 

 This indicates that the cutaneous arterioles are dilated, and 

 this dilatation has doubtless a great deal to do in many 



* R. Mever, Die organische Bewegung in ihrem Zusammenhang mit dem 

 Stoffwechsel, 1845, p. 84. Meyer's explanation of the occasional red colour of 

 venous blood is ditlerent from the one I have given above. We both agree that 

 the slightness of the alteration it has undergone in its passage from the arteries 

 into the veins is due to the fact tliat but little oxygen has been taken from it by 

 the tissues as it flowed tlirough the capillaries. Meyer considers that the tissues 

 adapt tliemselves to the wants of the body, and take little oxygen from the blood 

 when the external air is warm. The oxidation which usually goes on within the 

 body is thus diminished, the production of the heat lessened, and the tempera- 

 ture of the animal prevented from rising too high. This hypothesis, though very 

 plausible, is rendered improbable by the experiments of Bernard {Revue 

 Scientijique, 1871-72, pp. 123 and 182), which show that the tissues of animals 

 whicli have been exposed to a high temperature absorb oxygen (after death at 

 least) much more quickly tlian usual. I therefore attribute the florid colour of 

 the blood to dilatation of the artei'ies and capillaries, allowing it to flow so 

 quickly through them tluit the tissues have not time to abstiacr much oxygen 

 however great their avidity for it may be. 



f Crawford, ]£xperiments and Observations on Animal Heat, 1788, p. 308. 



