T1IK BLOOD 35 



man. In the small arteries, veins, and larger capillaries they 

 an- seen going along two, three, or more abreast ; in the smaller 

 apillaries they follow each other in single file. When 

 they pass into a small branch they bend and change 

 their shape, and some may be seen flattened and elongated 

 as they are being squeezed through the smallest capillaries ; 

 directly they get room, they recover their proper shape 

 again. Colourless corpuscles, which are smaller than the 

 red corpuscles, being about the same size as the colourless 

 corpuscles of man, will also be seen. In the small arteries 

 the red corpuscles will be carried swiftly along in the centre or 

 axis of the vessel, while the colourless corpuscles, being 

 lighter and more adhesive, will be carried along more slowly 

 at the side of the stream. The colourless corpuscles are often 

 seen, especially in the capillaries and small veins, to stick to 

 the wall of the vessel for a time and then to be carried on again. 



The oxygen and nourishment which the tissues require is 

 constantly passing from the blood through the walls of the 

 capillaries, and similarly the waste matters which the tissues 

 form pass through the walls into the blood. 



Before we consider the course the blood takes in its circula- 

 tion, or the means by which its circulation is carried on, it will 

 be well to consider the build and movements of the body, and 

 the nature of some of the tissues. 



