96 THE CHEMICAL COMPOSITION OF THE BODY 



characteristics. A number of proteoses and peptones have been described. 

 However, there is no sharp dividing line between the more simple proteoses 

 and more complex peptones, or between the simple peptones and the peptides. 

 (The term peptide as at present understood designates only those combina- 

 tions of amino acids possessing a known definite structure.) The peptones 

 differ from the proteoses in being more diffusible, being non-precipitable 

 on saturation with ammonium sulphate, and by their failure to give certain 

 protein reactions. As a class, proteoses and peptones are relatively very 

 soluble and are non-coagulable by heat. 



Melanins are the pigmentary substances found in the hair, feathers, skin, 

 the choroid coat of the eye, and in some tumors. Products similar to the 

 naturally occurring melanins are obtained on hydrolizing nearly all proteins 

 with acids. The melanins are sulphur- containing acid-like substances, and 

 seem to be combinations of amino-sugars (glucosamine) with certain amino- 

 acids, especially tyrosine, tryptophane, and lysine. Iron is found in some of 

 the melanins. 



THE FATS. 



Fats occur very widely distributed in the plant and animal kingdom, and 

 constitute one of the four classes of food stuffs. Fats are esters or ethereal 

 salts consisting of an organic radical (glycerol) united with the residue of an 

 organic acid. Ethyl alcohol may be combined as an ester with acetic acid. 



CH 3 COOH + C 2 H 5 OH = CH 3 COOC 2 H 5 + H 2 O 



Acetic acid Alcohol Etyhl acetate 



Similarly the triatomic alcohol glycerol may be combined with the 

 higher fatty acids to form the true fats. 



CH 2 OH CH 2 -OOCC 15 H 31 



CHOH + 3 C 15 H 31 COOH = CH- OOCC 15 H 31 + 3 H 2 O 



CH 2 OH CH 2 -OOCCi 5 H 31 



Glycerol Palmitic acid Tri-palmitin 



The animal fats are for the most part mixtures of tri-palmitin, tri- stearin, 

 and tri-olein, the last two being esters of glycerol with stearic acid, C 17 H 35 

 COOH, and, with the unsaturated oelic acid, C 17 H 33 COOH. Human fat 

 consists of a mixture of which tri-palmitin and tri-stearin comprise three- 

 fourths of the whole. The fat in milk and butter is in part tri-butyrin, the 

 ester of glycerol with butyric acid, C 2 H 5 COOH. The percentage of any 

 individual fat in animal tissue depends on, and is characteristic of, the particu- 

 lar species of animal from which the fat was obtained. Ordinary mutton fat 

 contains more tri-stearin and less tri-olein than pork fat, and the mutton 

 fat is stiffer because the melting-point of the tri-stearin is the highest of 

 the fats. 



The pure fats are odorless, tasteless, and generally colorless. They 



