346 



OF THE CIRCULATION OF THE BLOOD. 



than is seen in the generality of the last-named parts, in virtue of the 

 peculiar activity of the molecular changes which take place in them. But 

 the arrangement of vessels peculiar to each, evidently has reference only 

 to the convenience of the distribution of blood among the elementary parts 



FIG. 152. 



FIG. 153. 



ssaifiw8^^^r? 



Distribution of Capillaries on the surface 

 of the Skin of the finger. 



Distribution of Capillaries around fol- 

 licles of Mucous Membrane. 



of the tissue, and varies with their form. It is not possible to imagine that 

 it has any other relation than this to their functions ; since the function of 

 each separate element of the organ, of which that of the entire organ is 



FIG. 154. 



i. 155. 



Distribution of Capillaries in Musclt 



Capillary network around Fat-cells. 



the aggregate, is due to its own inherent vital powers, the supply of blood 

 being only required as furnishing the material on which these are to be 

 exercised. 



262. The average rate of movement of the blood through the Capillary 

 system, may be determined with tolerable precision by microscopic measure- 

 ment ; and the observations of Hales, Valentin, and Weber concur in repre- 

 senting it to average in the systemic capillaries of the Frog 1.2 inch per 

 minute. In warm-blooded animals, however, the capillary circulation is 

 probably much more rapid than this; the observations of Volkmann upon 

 the mesenteric arteries of the Dog making its rate about 1.8 inch per min- 

 ute ; whilst Ludwig and Vierordt, from observations on the movement of 

 the blood-corpuscles in the retinal capillaries of their own eyes, estimate the 

 rapidity at from one inch in 41 seconds to one inch in 28 seconds. The 

 layer which is in immediate proximity to the wall of the vessels flows from 

 9 to 17 times more slowly if the movement of the white corpuscles is to be 

 taken as a means of estimating it (Weber). Assuming .0,'] inch per second, 

 however, as the rate, and comparing this with the rate of movement of the 

 blood in the larger arteries, which seems on the average to be 11.8 inches 

 per second, it is calculated by Volkmann that the aggregate area of the 

 capillaries (being in an inverse ratio to the rate of the blood's movement 

 through them) must be nearly four hundred times that of the arterial trunks 



