999 ANIMAL PHYSIOLOGY. 
If we suppose the epithelial lining pushed out of a small 
artery we have, so far as structure alone goes, a good idea of a 
capillary—i. e., its walls are but one cell thick, and these cells 
k ANY 
Fia. 200. Fie. 201. 
Fig. 200.—Vein with valves lying open (Dalton), 
Fig. 201.—Vein with valves closed, the blood passing on by a lateral branch below (Dalton). 
though long are extremely thin, so that it is quite easy to un- 
derstand how it is that the amoeboid corpuscles can, under cer- 
Fia. 202,—Capillary blood-vessels (Landois). The cement-substance between the endothelium: 
has been rendered dark by silver nitrate, and the nuclei made prominent by staining. 
tain circumstances, push their way through its probably semi- 
fluid walls. 
From what has been said, it will be seen that the whole col- 
lection of vascular tubes may be compared to two inverted fun- 
