90 ANATOMY FOR NURSES. [CHAP. VIII. 



Should the blood in its onward course towards the heart be, for 

 any reason, driven backwards, the refluent blood, getting be- 

 tween the wall of the vein and the flaps of 

 the valve, will press them inwards until 

 their edges meet in the middle of the chan- 

 nel and close it up. The valves have 

 usually two flaps, sometimes one, and 

 rarely three. The veins, like the arteries, 

 are supplied with both blood-vessels and 

 nerves, the supply, however, being far less 

 abundant. 



The capillaries. The walls of the cap- 

 illaries are formed entirely of a layer of 

 pairs of valves; B, longi- simple epithelium composed of flattened 



tudinal section of vein, ,, . . , 1 11 u 



showing valves closed. cells joined edge to edge by cement sub- 

 stance, and continuous with the layer 



which lines the arteries and veins. The capillaries commu- 

 nicate freely with one another and form interlacing networks 

 of variable form and size in the different tissues. Their 

 average diameter is so small that only two or three blood- 

 corpuscles can pass through them abreast, and in many parts 

 they lie so closely together that a pin's point cannot be in- 

 serted between them. They are most abundant, and form the 

 finest networks in those organs where the blood is needed for 

 other purposes than local nutrition, such as, for example, for 

 secretion or absorption. In the glandular organs they supply 

 the substances requisite for secretion; in the intestines they 

 take up the elements of digested food ; in the lungs they absorb 

 oxygen and give up carbonic acid ; in the kidneys they discharge 

 the waste products collected from other parts ; all the time, every- 

 where through their walls, that interchange is going on which is 

 essential to the renovation, growth, and life of the whole body. 

 It must be remembered that although the arteries, veins, and 

 capillaries have each the distinctive structure above described, 

 it is at the same time difficult to draw the line between the 

 smaller artery and larger capillary, and between the larger 

 capillary and smallest vein. The veins on leaving the capillary 

 networks only gradually assume their several coats, while the 

 arteries dispense with their coats in the same imperceptible way 

 as they approach the capillaries. 



