CHAP. IX.] THE VASCULAR SYSTEM. 113 



them inwards until their edges meet in the middle of the chan- 

 nel and close it up. The valves have usually two flaps, some- 

 times one, and rarely three. The veins, like the arteries, are 

 supplied with both blood-vessels and nerves, the supply, how- 

 ever, being far less abundant. 



The capillaries. The walls of the capillaries are formed 

 entirely of a layer of simple endothelium composed of flat- 

 tened cells joined edge to edge by cement substance, and 

 continuous with the layer which lines the arteries and veins. 

 The capillaries communicate freely with one another and form 

 interlacing networks of variable form and size in the different 

 tissues. Their diameter is so small that often the blood-cor- 

 puscles must pass through them in single file, and in many parts 

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

 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 sub- 

 stances requisite for secretion; in the alimentary canal 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. 



Serous membranes. Serous membranes are thin and transparent, tol- 

 erably strong, extensile, and elastic. They are lined on the inner surface by 

 a simple epithelial layer of flattened cells (endothelium). The surfaces are 

 moistened by a fluid resembling serum, and from which the membranes 

 obtain their name of serous membranes. Here and there between the cells 

 openings are seen, which are of two kinds. The smaller and more numerous 

 are false openings, and are termed pseudo-stomata; the larger or true aper- 

 tures are termed stomata, and open into subjacent lymphatics. The sub- 

 stance of serous membranes underneath the endothelium is composed of a 



