CIRCULATION, 81 



under the microscope, we find, in addition to a fluid, which 

 is called plasma, there are numerous small discs, or corpuscles, 

 floating about in the fluid. 



There are two varieties of blood corpuscles, the red and the 

 white. It is the presence of the enormous quantity of red 

 corpuscles that gives to the blood its red color. They are 

 little, flat, circular discs, resembling a coin, only thicker 

 near the rim than at the centre. They have a strong ten- 

 dency to run together, like a roll of ten-cent pieces as 

 seen in Fig. 25, where some lie separate, while others are 

 in -rolls. 



The white corpuscles are not so numerous, only about one 

 to every four hundred of the red. They are a little larger 

 and more globular in shape, although, watched under the 

 microscope, it will be noticed that in making their way 

 through the minute vessels they change their shape. 



2. Uses of the Blood. These little corpuscles are really 

 the carriers of food to the tissues. Like boats on a stream, 

 they float along, laden with material, which thoy unload into 

 the system ; then reload with the refuse, and carry it back to 

 the lungs, to be given off into the air. They are charged with 

 oxygen in the lungs, and carry it to where there is work to 

 be done or repairs to be made. 



The plasma is rich in mineral matter for the bones, and in 

 albumen for the muscles. 



3. Clotting of Blood. When blood is drawn from the 

 body it soon clots, that is, it forms into a jelly-like mass. 

 The clot consists mainly of two substances a network of 

 tough, fibrous threads, called fibrin, which separates out from 

 the plasma and the corpuscles, which are caught in this net- 

 work. The clotting of the blood is an important provision of 

 nature for arresting its flow from a wounded blood-vessel. 

 The moment blood escapes from the vessel, the fibrin begins 

 to form, and clogs up the cut and arrests the flow from th5 



6 



