84 ELEMENTS OF PHYSIOLOGY 



must be liquid, hence a large part of it is water. It is the 

 plasma or watery portion of the blood which transports 

 the food to the tissues. The plasma is not, then, a simple 

 but a very complex liquid. The tissues require albumin, 

 fat, sugar, and mineral food. We should expect the plasma 

 to contain all of them, and so it does. As we should like- 

 wise expect, it gets this nourishment from the alimentary 

 canal. The sugar is in the form of grape sugar, and be- 

 cause of this, blood or meat containing it is slightly sweetish 

 in taste. The fat is in the form of minute globules, and 

 the albumin is similar to egg albumen, but very much 

 diluted with water. 



138. Let us take up another function of the blood. How 

 does it carry the oxygen ? This gas enters the red cor- 

 puscles while the blood is in the lungs, and makes them a 

 brighter red. It is given up to the tissues ; and at the same 

 time the tissues give up carbon dioxid, which is carried to 

 the lungs by the corpuscles and the plasma. YtTe liquid 

 impurities in the tissues are carried by the plasma to the 



j skin and kidneys. We see, then, that the 



i Jj red corpuscles transport the greater part 



of gaseous material, while the plasma 

 I ^Hak transports the liquids and the solid mate- 

 mm fl H r ^ s dissolved m the liquids. 



139. What are the White Corpuscles for? 



\^&/ -This brings us to a fourth function or 



FIG 81 use f tne blood, a function that has been 



i, human blood cor- discovered only within the last few years. 



fronfvkwf t fro"* The blood of healthy persons possesses a 



biood corpuscles, cer tain power of destroying germs of dis- 



side and front view. ' c 



Both are drawn to ease which may enter the body; this is 



the same scale. - . . 



called its germicidal or germ-killing power. 

 The germs are minute one-celled organisms to be seen 

 only through a powerful microscope. Some float in air ; 

 others are found in liquids or solids. Most kinds of germs 



