THE ALBUMINOIDS. 



505 



3. Keratin occurs in all horny and epidermic tissues (epidermic scales, hairs, 

 nails, feathers)- C. 50'3-52-S ; H. 6-4-7 ; N. 16-2-177 ; O. 20-8-25 ; S. 07-5 per 

 cent., is soluble only in boiling caustic alkalies, but swells up in cold concentrated 

 acetic acid. When decomposed by H 2 S04 it yields 10 per cent, leucin and 3 '6 per 

 cent, tyrosin. 



4. Fibroin is soluble in strong alkalies and mineral acids, in ammonio- 

 sulphate of copper ; when boiled with H 2 S0 4 it yields 5 per cent, tyrosin, leucin, 

 and glycin. It is the chief constituent of the cocoons of insects and threads of 

 spiders. 



5. Spongin, allied to fibroin, occurs in the bath-sponge, and yields as decom- 

 position products, leucin and glycin (Stadeler). 



6. Elastin, the fundamental substance in elastic tissue, is soluble only 

 when boiled in concentrated caustic potash (C. 55-55 '6 ; H. 7-1-77 ; N. 16-1-177; 

 0. 19 '2-21 ! per cent. ) It yields 36-45 per cent, of leucin and | per cent, of tyrosin. 



7. Gelatin, obtained from connective-tissues by prolonged boiling with 

 water; it gelatinises in the cold (C. 52-2-507; H. 6'6-7'2; N. 17'9-18'8; 

 S. +0, 23*5-25 ; (.S. 0'6 per cent.). [The ordinary connective-tissues are supposed 

 to contain the hypothetical anhydride collagen, while the organic basis of bone is 

 called ossein.] It rotates the ray of polarised light strongly to the left. By pro- 

 longed boiling and digestion it is converted into a peptone-like body (gelatin- 

 peptone), which does not gelatinise (p. 332). [It swells up, but does not dis- 

 solve in cold water ; when dissolved in warm water and tinged with Berlin blue 

 or carmine it forms the usual coloured mass which is employed by histologists for 

 making fine transparent injections of blood-vessels.] A body resembling gelatin 

 is found in leukseinic blood and in the juice of the spleen (p. 206). When decom- 

 posed with sulphuric acid it yields glycin, ammonia, leucin, but no tyrosin. It 

 gives insoluble precipitates with mercuric chloride and tannin. 



8. Chondrin ( Joh. M tiller) occurs in the matrix of hyaline cartilage and between 

 the fibres in fibre-cartilage. It is obtained from hyaline cartilage and the cornea 

 by boiling. It occurs also in the mantle of molluscs (C. 49 '5-50 '9; H. 6-6-7 '1 ; 

 N. 14-4-14-9; S + O. 27 '2-29; S. 0'4 per cent.). When boiled with sulphuric 

 acid it yields leucin; with hydrochloric acid, and when digested chondro-glucose 

 (Meissner) ; it belongs to the glucosides, which contain N. When acted upon by 

 oxidising reagents it is converted into gelatin (Brame). The substance which 

 yields chondrin is called chondrogen, which is perhaps an anhydride of chondrin. 

 The following properties of gelatin and chondrin are to be noted: 



