CH. XXVIII.] THE PHOSPHO-PROTEINS 429 



the skeletal origin and often insoluble nature of the members of the 

 group. The principal proteins under this head are : 



Collagen, the substance of which the white fibres of connective 

 tissue are composed. Some observers regard it as the anhydride of 

 gelatin. In bone it is often called ossein. 



Gelatin. This substance is produced by boiling collagen with 

 water. It possesses the peculiar property of setting into a jelly when 

 a solution made with hot water cools. On digestion it is like ordinary 

 proteins converted into peptone-like substances, and is readily 

 absorbed. Though it will replace in diet a certain quantity of such 

 proteins, acting as what is called a " protein-sparing " food, it cannot 

 altogether take their place as a food. Animals whose sole nitrogenous 

 food is gelatin waste rapidly. The reason for this is that "gelatin 

 contains neither the tyrosine or the tryptophane groups, and so it 

 gives neither Millon's nor the Adamkiewicz reactions. Animals who 

 receive a mixture of gelatin, tyrosine, and tryptophane in their diet 

 thrive better. 



Chondrin is the name given to the mixture of gelatin and mucoid 

 which is obtained by boiling cartilage. 



Elastin. This is the substance of which the yellow or elastic 

 fibres of connective tissue are composed. It is a very insoluble 

 material. The sarcolemma of muscular fibres and certain basement 

 membranes are very similar. 



Keratin, or horny material, is the substance found in the surface 

 layers of the epidermis, in hairs, nails, hoofs, and horns. It is very 

 insoluble, and chiefly differs from most other proteins in its high 

 percentage of sulphur. A similar substance, called neurokeratin, is 

 found in neuroglia and nerve-fibres. In this connection it is interest- 

 ing to note that the epidermis and the nervous system are both 

 formed from the same layer of the embryo the epiblast. 



6. The Phospho -proteins. 



Vitellin (from egg-yolk), caseinogen, the principal protein of 

 milk, and casein, the result of the action of the rennet-enzyme 

 on caseinogen (see milk), are the chief members of this group. 

 Among their decomposition products is a considerable quantity of 

 phosphoric acid. They have been frequently confused with the 

 nucleo-proteins, and the prefix nucleo so often applied to them 

 is entirely misleading, since they do not yield the products (purine 

 bases, etc.) which are characteristic of nucleo-compounds. The 

 phosphorus is contained within the protein molecule, and not in 

 another molecular group united to the protein, as is the case in the 

 nucleo-proteins. The phospho-proteins are specially valuable for the 

 growth of young and embryonic animals. 



