THE PROTEIDS 31 



C. The Albuminoids 

 The albuminoids form a heterogeneous group of substances which, 

 though similar to the proteids in many particulars, differ from them 

 in certain other points. The principal members of the group are the 

 following : — 



1. Collagen, the substance of which the white fibres of connective 

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

 gelatin. 



2. Ossein. — This is the same substance derived from bone;^ 



3. 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. It gives most of the proteid 

 colour tests. Many observers state, however, that it contains no 

 sulphur. On digestion it is like proteid converted into peptone-like 

 substances, and is readily absorbed. Though it will replace in diet 

 a certain quantity of proteid, acting as what is called a * proteid- 

 sparing ' food, it cannot altogether take the place of proteid as a food. 

 Animals fed on gelatin instead of proteid waste rapidly. Chondrin, 

 the very similar substance obtained from hyaline cartilage, appears 

 to be a mixture of gelatin with mucinoid materials. 



4. 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 ai"e very similar. 



5. 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 proteids in its high 

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

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

 esting to note that the epidermis and the nei-vous system are both 

 formed from the same layer of the embryo — the epiblast. 



6. Chitin and similar substances found in the exo-skeleton of 



many invertebrates. 



' In round numbers the solid matter in bone contains two-thirds inorganic or 

 earthy matter, and one-third organic or animal matter. The inorganic constituents 

 are calcium phosphate (84 per cent, of the ash), calcium carbonate (13 per cent.), 

 and smaller quantities of calcium chloride, calcium fluoride, and magnesium 

 phosphate. The organic constituents are ossein (this is the most abundant), 

 elastin from the membranes lining the Haversian canals, lacunae, and canaJiculi, 

 and proteids and nuclein from the bone corpuscles. There is also a small quantity 

 of fat even after removal of all the marrow. Dentine is like bone chemically, but 

 the proportion of earthy matter is rather greater. Enamel is the hardest tissue in 

 the body ; the mineral matter is like that found in bone and dentine ; but the 

 organic matter is so small in quantity as to be practically non-existent. (Tomes.) 

 Enamel is epiblastic, not mesoblastic like bone and dentme. 



