CIRCULATION OF THE BLOOD IN THE CAPILLARIES. 81 



to the condition of the arterioles which are supplied by it. When these little vessels are 

 dilated, the artery of course empties itself with greater facility, and the rapidity is in- 

 creased. Thus the rapidity bears a relation to the arterial pressure ; as, independently of 

 a diminution in the entire quantity of the circulating fluid, variations in the pressure 

 depend chiefly on causes which facilitate or retard the flow of blood into the capillaries. 

 A good example of enlargement of the capillaries of a particular part is in mastication, 

 when the salivary glands are brought into activity and the quantity of blood which they 

 receive is greatly increased. Chauveau found an immense increase in the rapidity of the 

 flow in the carotid of a horse during mastication. The enlargement of the vessels 

 of the glands during their function has been conclusively proven by the experiments of 

 Bernard. It must be remembered that, in all parts of the arterial system, the rapidity 

 of the current of blood is constantly liable to increase from dilatation of the small ves- 

 sels and to diminution from their contraction. 



Circulation of the Blood in the Capillaries. 



Before entering upon the study of the capillary circulation, we should define what we 

 mean by the capillary vessels as distinguished from the smallest arteries and veins. From 

 a strictly physiological point of view, the capillaries are to be regarded as commencing 



FIG. 27. Capillary blood-vessels from the pecten of the eye of the bird. (Eberth.) 



a, small capillaries, with fusiform cells; &, capillaries with polygonal cells; ft', hyaloid membrane investing the capil- 

 laries; c, capillaries from the intestine of the snail. 



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