6 ELEMENTARY PHYSIOLO(JY less. 



chemists as Protein matter (Losson IIT.), and which con- 

 tains carbon, hydrogen, oxygen, nitrogen, and sulphur, 

 must form a part of this food, if it is to sustain life in- 

 definitely ; and fatty, starchy, or saccharine, i.e. carbo- 

 hydrate matters, together with a certain amount of salts, 

 ought to be contained in the food, if it is to sustain life 

 conveniently. 



A certain proportion of the matter taken in as food 

 either cannot be, or at any rate is not, used ; and leaves 

 the body, as excrement itions matter, having simply passed 

 through the alimentary canal without undergoing much 

 change, and without ever being incorporated into the 

 actual substance of the body. But, under healthy con- 

 ditions, and when only so much food as is necessary is 

 taken, no important proportion of either protein matter, 

 or fat, or starchy or saccharine food, passes out of the 

 body as such. Almost all real food ultimately leaves the 

 body as waste in the form either of water, or of 

 carbonic acid, or of a third substance called urea, or 

 of certain saline compounds or salts. 



Chemists liave determined that these products which are 

 tlirown out of the body and are called excretions, con- 

 taui if taken altt)gether, far more oxygen than the food 

 and water taken into the body. Now, the only possible 

 source whence the body can obtain oxygen, except from 

 food and water, is the air which surrounds it.i And care- 

 ful investigation of the air which leaves the chamber in 

 the imaginary experiment described above would show, 

 not only that it has gamed carbonic acid from, the man, 

 but that it has lost oxygen in equal or rather greater 

 amount to him. 



Thus, if a man is neither gaining nor losing weight, the 

 sum of the weights of all the substances above enumerated 

 which leave the body ought to be exactly equal to the 



1 Fresh country air contains in every 100 parts nearly 21 of oxygen and 

 79 of nitrogen gas, together with a small fraction of a part ('04) of car- 

 bonic acid, and a variable quantity of watery vapour. The constitvient 

 of the atmosphere, argon, present in small quantities, is here reckoned 

 in with the nitrogen. (See Lesson IV.) 



