226 ANIMAL PHYSIOLOGY. 
THE CIRCULATION UNDER THE MICROSCOPE. 
There are few pictures more instructive and impressive than 
a view of the circulation of. the blood under the microscope. 
It is well to have similar preparations, one under a low power 
and another under a magnification of 300 to 500 diameters. With 
the former a view of arterioles, veins, and capillaries may be 
Fig. 205.— Portion of the web of a frog's foot as seen under a low magnifying power, showing 
the blood-vesselsi and in one corner the pigment-spots (after Huxley). a, small arteries 
(arterioles); v, sthall veins. The smaller vessels are the capillaries. The course of the 
blood is indicated by arrows. 
obtained at once. Many different parts of animals may be used, 
as the web of the frog’s foot, its tongue, lung, or mesentery ; 
the gill or tail of a small fish, tadpole, etc. 
The relative size of the vessels; the speed of the blood-flow; 
the greater velocity of the central part of the stream; the aggre- 
gation of colorless corpuscles at the sides of the vessels, and the 
occasional passage of one through a capillary wall, when the 
exposure has lasted some time; the crowding of the red cells; 
their plasticity ; the small size of some of the capillaries, barely 
