CHAP. XXVIII.] THE CIRCULATION IN THE CAPILLARIES. 367 



rate of movement of the blood as it flows from trunks to branches. 

 But the careful measurements of Mr. Fernaby and of Mr. Paget, 

 render it necessary to modify the general proposition to some extent. 



Mr. Fernaby * compared the areas of trunks and branches in 

 the only sound way, namely, according to the geometrical law, that 

 the areas of circles are as the squares of their diameters. Esti- 

 mated thus, he found that the excess of the combined areas of 

 the branches over those of the trunks was very trifling, and, in 

 some instances, scarcely appreciable; and Mr. Paget, while con- 

 firming the general statement of Mr. Fernaby, discovered a re- 

 markable exception in the case of the common iliac arteries, whose 

 combined areas were distinctly less than that of the aorta above the 

 point of bifurcation and the combined areas of the external and 

 internal iliacs were less than that of the common but those of the 

 branches of the external iliac exceeded notably the area of their 

 parent trunk.f 



Volkmann states, that in general the arterial capacity is increased 

 in area by the division into branches. But he instances a remarkable 

 exception, in the case of the external and internal carotids of the 

 horse, whose combined areas are smaller than that of the trunk. 

 He remarks, likewise, that the first divisions of the larger arterial 

 trunks (aorta and pulmonary artery) experienced very little increase 

 of area ; but that, as subdivision goes on, the increase becomes 

 much more marked. And it is especially so near the capillaries, 

 where the combined areas of some small branches nearly double 

 that of their parent. This is particularly interesting, as denoting 

 the coincidence of physical conformation with the result of direct 

 observation, on the velocity of the blood, which shew that it is 

 near the capillaries, that the most decided diminution takes place 

 in the rate of the blood's motion. 



Of the Circulation in the Capillaries. The manner in which the 

 blood flows through the capillaries is easily made the subject of 

 direct observation, by examining the transparent parts of certain 

 animals, as the wings of bats, the mesenteries of small animals as 

 the mouse, the web of the frog's foot, the lung of the frog, or of 

 the newt, etc. 



In watching the circulation in the web of the frog's foot under 



the microscope, with a magnifying power of about 200 diameters, 



the following points will attract observation : first, it will be seen 



that the stream is continuous, that is, it rushes with a considerable 



* Lond. Med. Gaz. 1839. t Lond. Med. Gaz. 1842. 



