40 HUMAN PHYSIOLOGY 



soluble in water; in general, it is not soluble without previous 

 decomposition. It cannot be digested. 



2. Elastin, from the elastic fibres, has only firmly combined 

 sulphur. It gives the color reactions of proteids and yields the 

 corresponding decomposition products. It is insoluble in water 

 and can be digested by pancreatic juice, not by gastric juice. 



3. Collagen forms the greater part of the albuminoids 

 found in our body. It forms the fibres of the connective 

 tissue and the organic basis of bones and cartilages. It 

 contains no loosely combined sulphur, does not give Adam- 

 kiewicz's and Miilon's reactions, neither does it yield tyrosin 

 when decomposing, hence it contains no aromatic group. 

 It contains a little more oxygen than the proteids, hence it 

 is perhaps formed from proteids by oxidation. 



If collagen is boiled for a long time in water, it takes up 

 water and, on cooling, a solid jelly called gelatin is formed. 

 If gelatin is heated to 130 it again changes to collagen. 

 Gelatin is the hydrate of collagen. It is soluble in hot but 

 not in cold water (the reverse of native proteids) ; in cold 

 water it only swells up. 



'Gelatin is not precipitated by mineral acids, by potassium 

 ferrocyanide and acetic acid, nor by salts of heavy metals 

 (except mercuric chloride in hydrochloric acid). It is, 

 however, precipitated by salts. It gives the biuret and 

 xanthoproteic test. 



Gelatin is digested with difficulty by pepsin but readily by 

 trypsin, gelatoses and gelatin peptones being formed. 

 Concerning the importance of gelatin as a food see Chapter 

 VIII. 



4. Chondrin is a mixture of gelatin and chondroitin-sulphuric 

 acid which is an ethereal sulphate of chondroitin, a nitrogenous 

 derivative of carbohydrates; it can be isolated from cartilage by 

 dilute alkalies. If bones are boiled with dilute mineral acids, the 

 chondroitin yields acetic acid and the nitrogenous chondrosin. 

 Chondrosin reduces cupric oxide in alkali solutions, and by boil- 

 ing with barium hydrate yields glycuronic acid and glucosamine. 



Ferments. Among the proteid-like bodies are also 

 counted the unformed ferments. The composition of the 



