50 MATERIAL COMPOSITION OF MAN. 



it blood casein, and Henle 1 thinks it probable, that it is in reality only 

 albumen with the membranes of the blood corpuscles. Berzelius con- 

 siders the crystalline lens to be composed of the same substance. 



3. Pepsin. This substance, to which Eberle gave the name, was dis- 

 covered by Schwann. It seems to be a modification of protein, but has 

 not been much examined. It is contained in the gastric juice ; and its 

 physiological properties will be described under the head of DIGESTION. 

 It greatly resembles albumen ; coagulates by heat and alcohol; and loses 

 its solvent virtues. It is best procured by digesting portions of the 

 mucous membrane of the stomach in cold water, after they have been 

 macerated for some time in water at a temperature between 80 and 

 100 of Fahrenheit. The warm water dissolves various substances as 

 well as some of the pepsin ; but the cold water takes up little more than 

 the pepsin, which is obtained by evaporating the cold solution in the form 

 of a grayish-brown viscid fluid. The addition of alcohol throws down 

 the pepsin in grayish-white flocculi; and one part of the principle thus 

 prepared, when dissolved in even 60,000 parts of water, will digest 

 meat and other alimentary substances. Liebig doubts the existence of 

 pepsin as a distinct compound. According to him as explained here- 

 after the solvent power of the gastric juice is owing to the gradual 

 decomposition of a matter dissolved from the lining membrane of the 

 stomach, aided by oxygen introduced into the saliva. 



4. G-elatin. This is the chief constituent of cellular tissue, skin, 

 tendons, ligaments, and cartilages. The membranes and bones also 

 contain a large quantity of it. It is obtained by boiling these sub- 

 stances for some time in water ; clarifying the concentrated solution ; 

 allowing it to cool, and drying the substance, thus obtained, in the air. 

 In this state it is called glue; in a more liquid form, jelly. Gelatin 

 dissolves readily in hot water ; is soluble in acids and alkalies ; insolu- 

 ble in alcohol, ether, and in fixed and volatile oils. Alcohol precipi- 

 tates it from its solution in water. It is not a compound of protein : 

 hence it has been concluded, that it cannot yield albumen, fibrin, or 

 casein; and, therefore, that blood cannot be formed of it. -The animal 

 system, it has been maintained, can convert one form of protein into 

 another, but cannot form protein from compounds that do not contain 

 it. This deduction as stated hereafter is probably too hasty. It is 

 admitted, that gelatin may be produced from fibrin and albumen ; since, 

 in animals that are fed on these alone, the nutrition of the gelatinous 

 tissues does not seem to be impaired ; and it is as easy to conceive that 

 gelatin may go to the formation of the proteinaceous tissues. 



Gelatin, nearly in a pure state, forms the air-bag of different fishes, 

 and is well known under the name of isinglass. It is used extensively 

 in the arts, on account of its adhesive quality, under the forms of glue 

 and size. What is called portable soup is dried jelly, seasoned with 

 various spices. 



5. Ohondrin. This was first discovered by J. Miiller. It is obtained 

 by boiling the cornea, the permanent cartilages, and the bones before 

 ossification. It is a variety of gelatin. 



1 Op. cit., p. 53. 



