180 Biological Chemistry. 



complex than in animals, a far larger number of chemical 

 processes would be expected to take place in one tissue. 

 For this reason it is probable that an organ in mammals 

 with the most diversified chemical activity, such as the 

 liver, probably contains a less complex mixture of sub- 

 stances than, for example, the leaves of plants. Whether 

 this is actually so or not, biochemical knowledge is much 

 more extensive as far as it relates to the higher animals 

 than it is as far as it relates to plants. The latter, it is 

 true, contain a very large variety of chemical substances, 

 the chemistry of which has been studied in detail, such as 

 the alkaloids, the essential oils, tannins, etc., but so far 

 their biological significance is almost unknown. 



The chief constituents of the mammalian body fall 

 into three classes the fats, the carbohydrates, and the 

 proteins. The tissues of plants are also, to a very large 

 extent, made up of substances which fall into one of these 

 three classes. They form, therefore, not only the chief 

 constituents of the body of animals, but also of their food. 

 A knowledge of the chemistry of the substances belonging 

 to these three classes is, therefore, of primary importance 

 to the student of physiology. 



Part I. The Fats. 



A brief mention has been already made of the fats 

 and oils (p. 99). These substances are esters of the tri- 

 hydroxy alcohol, glycerol, in which all the hydroxyl 

 hydrogens are replaced by an acid radicle. Included in 

 the category of the fats are substances known as oils or 

 fats,* the former name being given to those products 

 which are liquid at ordinary atmospheric temperatures, 



* The substances known as "waxes " are generally esters of higher alcohols, 

 such as cetyl alcohol, CicH^O, which occurs in the form of cetyl palmitate in 

 spermacetti. 



