74 ANATOMY FOR NURSES. [CHAP. VII. 



Hence, by means of the lymph acting as middleman, a double 

 interchange of material takes place between the blood within 

 the capillary and the tissue outside the capillary. In every 

 tissue, so long as life lasts and the 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 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 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 bodies capable of being made use 

 of by some other tissue. 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 organism lives on the things around it, its 

 air and its food, so the several tissues live on the complex fluid 

 by which they are all bathed, and which is to them their imme- 

 diate 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. 



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, colourless fluid, with minute solid par- 

 ticles 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. 



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



