446 HEALTH AND DISEASE 



the heart beats more rapidly, indeed, but much more feebly ; and with faint- 

 ness and loss of consciousness the current of blood almost ceases. Finally, 

 the convulsions that are the precursors of death drive the few remaining 

 drops to the heart, and, by stimulating it to contract, afford the last chance 

 of life being preserved. 



The Capillaries. As the arteries proceed to the proper tissues of the 

 body, they divide and subdivide till they are no longer visible to the un- 

 assisted eye, and they finally break up into minute vessels named capillaries, 

 the size of which is tolerably uniform for each organ. These communicate 

 freely with each other and form a mesh or web, the arrangement of which 

 presents variations corresponding to the structure of the organ in which 

 they exist, being ladder-like in muscle, tuft-like in the kidney, and basket- 

 like round the lobules of fat and of glands. 



The wall of the capillaries is formed of a delicate basement membrane 

 with a lining of flattened cells (endothelium), or even of the cells alone. 

 Through this thin wall the white corpuscles of the blood seem to be able to 

 bore without leaving any rent behind them, just as a needle may be thrust 

 through the wall of a soap-bubble and withdrawn without causing the 

 bubble to burst. The corpuscles then become free, and can move about in 

 the interspaces of the adjoining tissues; but whether they die there or re- 

 enter the vessels is unknown. This process, in which the white corpuscles 

 pass out of the vessels, is named diapedesis. The current of blood is not 

 always the same in the same capillary vessel, the direction being dependent 

 upon the increase of pressure behind, that is, on the arterial side, or the 

 relief of pressure in front or on the venous side. In examining the circula- 

 tion in the capillaries it will be seen that they are not large enough in 

 many parts to admit two coloured corpuscles abreast, and they consequently 

 follow each other in single file; but in the somewhat larger vessels the red 

 corpuscles occupy the centre of the stream, whilst the white corpuscles roll 

 lazily along at the margin in close contact with the inner surface of the 

 wall of the vessel. It is in the play of the fluids within and without 

 the capillary vessels that an important part of the processes of nutrition 

 is transacted, the blood surrendering to the fluid which moistens their 

 external wall the soluble materials for the nutrition of the tissues, whilst 

 it takes up from that fluid the soluble products of the degeneration 

 and decay of the tissues. The capillaries of the lungs are very large, 

 and form a close net- work. Those of the brain are minute and less 

 close. The average diameter of the capillaries may be taken to be a 

 little larger than the diameter of the corpuscles of the blood of the same 

 animal. The pressure of the blood in the capillaries is considerable, being 

 capable of supporting a column of mercury about 1 inch in height. The 



