172 ELEMENTARY PHYSIOLOGY less. 



various substances which are the products of the activity 

 of the several tissues, muscles, brain, glands, &c., and 

 which pass from the tissues into the blood. We may 

 speak of these as waste products, and one of them which 

 is produced by all the tissues, namely carbonic acid, is 

 emphatically a waste product and is got rid of as soon as 

 possible. But some of the substances which are returned 

 to the blood from the tissues are not wholly useless 

 matters to be thrown off as rapidly as possible ; they are 

 capable of being used up again by some tissue or other. 

 Thus, as we shall see, the liver, at certain times at all 

 events, returns to the blood a certain quantity of sugar 

 which is made u.se of in other parts of the body, and 

 similarly the spleen, while it takes up certain substances 

 from the blood, gives back to the blood certain other 

 substances which we can hardly speak of as waste matters 

 in the sense of being useless material fit only to be at 

 once thrown away. 



In the second place, the blood is continually receiving 

 from the alimentary canal the materials arising from the 

 food which has been digested there. As we shall see, 

 some of this material passes directly from the cavity of the 

 alimentary canal into the blood, but some of it goes in a 

 more roundabout way through what are called the lacteals 

 or lymphatics. On its way to the blood this latter is 

 joined by material which, escaping from the blood and 

 not used by the tissues, or passing from the tissues directly 

 into the lymphatics, is carried back to the blood by the 

 thoracic duct (see Le.sson II., p. 87). 



In the third place, the blood is continually gaining 

 oxygen from the air through the lungs. 



Then again the blood, while it loses heat by the skin and 

 lungs, gains heat from the tissues. As we have already 

 seen (Lesson I., p. 25) oxidation is continually going on in 

 various parts of the body, and by this oxidation heat is 

 continually being set free. Some of this oxidation may 

 take place in the blood itself ; we do not know exactly how 



