72 



MANUAL OF PHYSIOLOGY. 



Groups III. ALBUMINOIDS. 



These are the outcome of nutritive modification of protoplasm, 

 and may be said to be directly manufactured by that substance, 

 and to be specially adapted to meet the requirements of certain 

 textures differing widely in function. They are allied to one an- 

 other and to the last group by (a) their percentage composi- 

 tion;* (b) containing nitrogen ; (c) being amorphous colloids. 

 They differ from albuminous bodies in (a) their solubility ; (b) 

 their behavior to heat, acids, alkalies, and the digestive fluids ; 

 and (c) their value as food stuffs. 



1. Mucin is the characteristic ingredient of the mucus manu- 

 factured by epithelial cells, and is also found in connective tissue 

 (abundantly in that of the foetus) and in some pathological 

 growths. It gives a peculiar thick ropy consistence to the fluid 

 containing it, enabling it to be drawn into threads. It is precip- 

 itated by mineral acids, alum and alcohol, and the precipitate 

 swells in water and is redissolved in excess of the acid. With 

 acetic acid a precipitate is formed which does not redissolve in 

 an excess of the acid. When boiled with sulphuric acid it yields 

 leucin and tyrosin. 



2. Chondrin is obtained by the prolonged boiling in water of 

 slices of cartilage cleared of the perichondrium. On cooling, this 

 solution forms a jelly. The jelly dissolves easily in hot water or 

 alkalies, and can be precipitated by acetic or weak mineral acids, 

 alum, or acetate of lead. It gives only leucin on boiling with 

 sulphuric acid. 



*The following table gives the composition of the principal albuminoids 

 and albumin : 



