232 THE BODY AT WORK 



upon the diminution in the rate at which blood flows through 

 the vessels. The velocity with which a stream flows through 

 a channel varies as the cross-section of the channel. In a 

 capillary vessel the blood flows at the rate of from 0-5 milli- 

 metre to 1 millimetre per second. In the aorta the velocity 

 is about 320 millimetres per second. In the re-formation of 

 the venous system a converse process of reduction occurs, but 

 not with anything like the same rapidity. The united calibre 

 of the two venee cavae, in which the reduction is complete, 

 is about twice that of the aorta. From this it follows that 

 the veins hold much more blood than the arteries ; and since 

 veins are more easily distended, the amount that they can 

 hold varies within wide limits. They constitute to some 

 extent a reservoir for blood. 



The capillary vessels are the tubes of the circulatory system 

 in which blood comes into use. On the average they are 

 about 0-5 millimetre long. Through them the blood flows 

 slowly. Through their walls alone is there any exchange worth 

 mentioning between the blood within the vascular system and 

 the lymph by which it is surrounded. Interest therefore centres 

 in these vessels. Their walls are formed of endothelial tiles. 

 In the centre of each thin transparent tile is a boss, where 

 its lens-shaped nucleus is situate. The outline of the tile is 

 sinuous. Its margin dovetails with the margins of those 

 adjacent to it. Oxygen and carbonic acid, nutrient sub- 

 stances and waste-products, pass rapidly through the endo- 

 thelial cells. Leucocytes have the power of pushing the cells 

 aside, in order that they may make their way out of the blood 

 into the lymph which fills the tissue-spaces. With the ex- 

 ception of the lens and cornea of the eye, cartilage, and the 

 various epidermal structures, all tissues are traversed by 

 capillary vessels. It is not difficult to calculate the number 

 of such vessels in the body exclusive of the liver and the lungs. 

 The diameter of the aorta is 28 millimetres, that of a capillary 

 about 0-008 millimetre. The cross-section of all the capillaries 

 added together is 640 times that of the aorta, as already stated. 



Many schemata have been devised to illustrate the vascular 

 system ; but all are misleading, inasmuch as they fail to give 

 any idea of the extent to which the subdivision of its vessels 

 is carried. If the water-pipes supplying a town branched 



