430 PRACTICAL PHYSIOLOGY 



to give them at all, so that their presence is held to depend on native 

 proteid in the gelatin. 



The other albuminoids are unimportant. They are Keratin, which 

 occurs in the skin appendages and in the medullary sheaths of nerves, 

 and which is remarkable for the large percentage of sulphur which it 

 contains ; Elastin found in elastic fibres, and which contains a very 

 small percentage of sulphur, and a considerable amount of aromatic bodies. 



All these albuminoids except keratin yield glycin as their chief 

 decomposition product. They also yield the hexone bases, since 

 protamin forms part of their molecule. 



III. True Proteids. (1) Native Proteids (p. 176). (2) Albuminates 

 (p. 177). (3) Proteoses and Peptones (p. 177). 



IV. Compound Proteids. (a) Gluco-Proteids. It has recently been 

 discovered that a carbohydrate can be split off from most proteids. 

 Thus, if egg albumin be decomposed by boiling with acid, the resulting 

 product is capable of reducing Fehling's solution although it does not 

 ferment with yeast. These reactions would point to its being a pentose 

 (see p. 417). A similar sugar is also obtained from the nucleo-proteid of 

 the pancreas and from several other proteids, so that the gluco proteids 

 probably form a very extensive class. Besides mucin (see p. 178), this 

 class also includes the Mucinoids and the Chondro Proteids. The 

 mucinoids are distinguished from the mucins in that they are not 

 stringy in nature, and that the}^ are not so easily precipitated by acetic 

 acid, and the precipitate is very readily soluble in excess of the acid. 

 They are represented by pseudo-mucin, which occurs in ovarian cysts, 

 and by ovo-mucoid in the white of egg. The chondro proteids occur 

 along with collagen in cartilage. On decomposition with an acid they 

 yield proteid and a reducing body called chondroitin-sulphuric acid, 

 which can be further decomposed to yield a body called chondrosin, 

 which is even more strongly reducing than dextrose, and which contains 

 nitrogen. By still further decomposing this latter body glucosamin is 

 obtained, which is also the chief decomposition product of chitin, 

 an important constituent of the carapace of arthropods. 



(b) Nucleins. The general relationships of the different bodies in 

 this most important group of proteids have already been given in the 

 elementary part (p. 179). All that remains to be done here is to 

 describe how the different bodies are prepared. 



Nucleo-Albumin and Nuclein. A pancreas or thymus gland is 

 chopped into small pieces, and is macerated for several hours with 

 about four times its bulk of water containing 0*5 per cent, ammonia, 

 the mixture being frequently stirred. After this time, the extract is 

 strained through muslin, and the residue again extracted. The extracts 



