78 



CIRCULATION OF THE BLOOD IN THE VESSELS. 



so marked, was found by Chauveau, between the carotid and the facial. The 

 last-named observer also noted an important modification in the character of 

 the current in the smaller vessels. As the vessels are farther and farther re- 

 moved from the heart, the systolic impulse becomes rapidly diminished, being 

 reduced in one experiment about two-thirds ; the dicrotic impulse becomes 

 feeble or may even be abolished ; but the constant flow is much increased in 

 rapidity. This fact coincides with the ideas already advanced with regard to 

 the gradual conversion, by reason of the elasticity of the vessels, of the im- 

 pulse of the heart into first, a remittent, and in the very smallest arteries, a 

 nearly constant current. 



The rapidity of the flow in any artery must be subject to constant modifi- 

 cations due 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 increased. Thus the rapidity bears a relation to 

 the arterial pressure ; as variations in the pressure depend chiefly on causes 

 which facilitate or retard the flow of blood into the capillaries. A good ex- 

 ample 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 a great increase in 

 the rapidity of the flow in the carotid of a horse during mastication. It must 

 be remembered that in all parts of the arterial system, the rapidity of the cur- 

 rent 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, it should be 

 distinctly stated what is meant by capillary vessels as distinguished from the 

 smallest arteries and veins. From a strictly physiological point of view, the 



capillaries are to be regarded as 

 beginning at the situation where 

 the blood is brought near enough 

 to the tissues to enable them to 

 separate the matters necessary 

 for their regeneration and to 

 give up the products of their 

 physiological wear ; but at pres- 

 ent it is impossible to assign 

 any limit where the vessels cease 

 to be simple carriers of blood, 



V*" H\M J$^ and it is not known to what 



part of the vascular system the 

 processes of nutrition are exclu- 

 sively confined. The divisions 

 of the blood-vessels must be, to 



FIG. 31. Capillary blood-vessels (Landois). . . .. , 



The boundaries of the cells (cement-substance between the a Certain extent, arbitrarily Q6- 



endotheliurn) is blackened with silver nitrate. The nu- c^j mu^ o^ ,-nA 



clei of the endothelium are brought out by staining. nneCl. 1116 HlOSt Simple, ana 



