96 ANATOMY FOR NURSES. [CHAP. VIII. 



blood flows through the blood-vessels, a fluid is passing from 

 the blood to the tissue, and from the tissue to the blood. The 

 fluid passing from the blood to the tissue carries to the tissue 

 the material which the tissue needs for building itself up and 

 for doing its work, including the all-important oxygen. The 

 fluid passing from the tissue to the blood carries into the blood 

 certain of the products of the chemical changes which have 

 been taking place in the tissue products which may be simply 

 waste, to be cast out of the body as soon as possible, or which 

 may be products capable of being made use of by some other 

 tissues. The tissues, by the help of the lymph, live on the blood, 

 and the blood may thus be regarded as an internal medium, 

 bearing the same relations to the tissue that the external 

 medium, the world, does to the whole individual. Just as the 

 whole body lives on the air and food around it, so do the several 

 tissues live on the complex fluid by which they are all bathed, 

 and which is to them their immediate air and food. 



The blood. The most striking external feature of the blood 

 is its well-known colour, which is bright red approaching to 

 scarlet in the arteries, but of a dark-red or purple tint in the 

 veins. It is a somewhat sticky liquid, a little heavier than 

 water, its specific gravity being about 1.055; it has a saltish 

 taste, a slight alkaline reaction, and a temperature of about 

 100 F. (37.8 C.). 



Seen with the naked eye the blood appears opaque and homo- 

 geneous; but when examined with a microscope it is seen to 

 consist of a transparent almost colourless fluid, with minute 

 solid particles immersed in it. The colourless fluid is named 

 plasma, the solid particles corpuscles. These corpuscles are of 

 two kinds, the red or coloured, and the white or colourless. In 

 a cubic millimetre 1 of healthy blood there are on an average 

 5,000,000 red corpuscles and 10,000 white. The number of 

 white varies much more than that of the red ; the proportion 

 of white to the red is usually given at from 1 to 250 up to 

 1 to 1000. 



Red corpuscles of the blood. The red corpuscles have a nearly 



circular outline like a piece of coin, and most of them have a 



shallow, dimple-like depression on both sides; their shape is, 



therefore, that of biconcave disks. The average size is ^Vo ^ 



1 A millimetre is equal to 0.039, or ^ of an English inch. 



