170 ALIMENTATION. 



botli animals and vegetables contribute nitrogenized matters. In both ani- 

 mal and vegetable food, nitrogenized substances are always found combined 

 with inorganic matters (water, sodium chloride, the phosphates, sulphates etc.), 

 and frequently with non-nitrogenized matters, especially the carbohydrates. 



The most important nitrogenized alimentary constituents of food are con- 

 tained in the muscular substance, eggs, milk, the juices of vegetables, cereal 

 grains etc. Many of these substances have been isolated and studied by 

 chemists. Among the most important are myosine, the chief organic con- 

 stituent of muscle, the various albumens found in eggs and in animal fluids, 

 analogous substances existing in vegetables, caseine in milk, a substance 

 sometimes called vegetable caseine, vitelline in yelk of egg, fibrin, gelatine, 

 and gluten, an important alimentary substance found in the cereal grains, 

 etc. A distinctive character of these substances is that they all contain nitro- 

 gen, being composed of carbon, oxygen, hydrogen and nitrogen, with prob- 

 ably a small quantity of sulphur. They are all either liquid or semi-solid in 

 consistence, not crystallizable, and are coagulable by various reagents. The 

 type of substances of this class is albumen, which has the provisional formula, 

 C 72 H 112 2 gN 18 S (Lieberkiihn) ; and they are sometimes called albuminoids. 

 They are also called proteids, after a hypothetical substance described by 

 Mulder, under the name of proteine. 



The nitrogenized substances are found in animal bodies, as has already 

 been stated. They originate in vegetables by a union of nitrogen, derived 

 from saline matters, with the carbohydrates, the carbohydrates in vegetables 

 being produced from carbonic acid and water. No part of the nitrogen used 

 by vegetables in the formation of the albuminoids is derived from the atmos- 

 phere (Hoppe-Seyler). 



A distinctive character of substances of this class is that under favorable 

 conditions of heat and moisture they undergo a peculiar form of decomposi- 

 tion, called putrefaction. In the process of digestion, these substances are 

 changed into peptones, and afterward, it is thought, into leucine, tyrosine 

 and some other substances not well defined. An analogous decomposition is 

 said to take place under the influence of dilute hydrochloric acid, at a tem- 

 perature of 104 Fahr. (40 C.), and of dilute sulphuric acid, at a tempera- 

 ture of 212 Fahr. (100 0.). The chemical history of these substances 

 would require for its comprehension an elaborate description such as proper- 

 ly belongs only to special works on physiological chemistry. 



Non- Nitrogenized Alimentary Substances. The important non-nitro- 

 genized alimentary substances are sugars, starch and fats. They are all com- 

 posed of carbon, hydrogen and oxygen. In sugars and starch, the hydrogen 

 and oxygen exist in the proportion to form water, and these matters are there- 

 fore called carbohydrates. The non-nitrogenized constituents of food are of 

 organic origin, definite chemical composition and crystallizable. 



Sugars. Many varieties of sugar occur in food, and this substance 

 may be derived from both the animal and the vegetable kingdoms. The 

 most common varieties derived from animals are sugar of milk, and honey, 

 beside a small quantity of liver-sugar, which is taken whenever the liver is 



