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MANUAL OF PHYSIOLOGY. 



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 manufac- 

 tured 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 precipitated by 

 mineral acids, alum, and alcohol, and the precipitate swells in 

 water and is re-dissolved in excess of the acid. With acetic acid 

 a precipitate is formed which does not re-dissolve in an excess of 

 the acid. When boiled with sulphuric acid it yields leucin and 

 ty rosin. 



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 sul- 

 phuric acid. 



3. Gelatin is produced by boiling fibrous connective tissues, such 

 as ligaments, tendons, the true skin and bones in water. On cool- 

 ing, the fluid forms a jelly, which can be dried to a colorless brit- 

 tle body which swells in cold water and dissolves on being heated. 

 It is not precipitated by acetic acid, but yields precipitates with 

 chloride of mercury or tannin, which latter is seen in making 

 leather. On boiling with sulphuric acid it yields glycin and leu- 

 cin but no tyrosiu. 



* The following Table gives the composition of the principal allmmi- 

 noids and albumin : 



