106 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



than the true albuminous bodies. They are termed gelatinous because 

 gelatin, which is formed by the action of boiling water on these tissues, 

 is the most important representative of the group. It contains — 



a. Collagen. 



b. Gelatin. 



c. Chondrogen. 



d. Chondral. 



a. Collagen, or gelatinous substance, forms the organic basis of 

 bones and teeth, and of the fibrous parts of tendons, ligaments, and 

 fascia. It derives its name from the fact that bj' prolonged boiling it is 

 converted into gelatin or glue (Ko/lXa). Collagen is prepared from bones 

 by soaking in repeated changes of dilute hydrochloric acid; or from 

 tendons b}- removing mucin by means of lime- or baryta-water, then by 

 repeated washing with water, and finally with very dilute acetic acid. 

 When fresh it is soft, but it shrinks and becomes hard when dry, or when 

 alcohol is added. Collagen is insoluble in cold water; it swells upon 

 dilute acids, and becomes transparent ; through the prolonged action of 

 dilute acids collagen dissolves, the solution containing gelatin and acid 

 albumen, the latter, perhaps, being produced by the action of the acid on 

 the residual matter of the connective-tissue cells. It dissolves in liquor 

 potassse, and in boiling dilute acids or in boiling water it dissolves and 

 is rapidly converted into gelatin. 



b. Gelatin. — Gelatin is prepared, as already indicated, by boiling 

 collagen, or any of the connective-tissue group, in water, and when the 

 solution cools it forms a jelly the' consistence of which depends upon 

 the percentage of gelatin present. 



Gelatin, prepared as indicated above, is a product of the trans- 

 formation of connective tissue by the prolonged action of boiling water. 

 It is favored by high temperature (120° C), as in Papin's Digester, and 

 the presence of a minute quantity of acid. When dry it forms a 

 yellowish or, if pure, a transparent, tasteless solid, closely resembling a 

 gum in its general appearance and characteristics. It is insoluble in 

 cold water, but when immersed in cold water is able to absorb by 

 imbibition forty times its own weight. It then will form a stiff, tenacious, 

 jelly-like mass. If dry gelatin is boiled in water, it is readily dissolved, 

 and when the solution in water cools the gelatin sets into a stiff jelly if 

 more than 1 per cent, of gelatin is present, the consistence of the jelly 

 depending upon the quantity of gelatin dissolved. When boiled for a 

 long time in water, or if boiled with an acid or alkali, this property of 

 gelatinizing is lost and two peptone-like bodies result. 



Gelatin is insoluble in alcohol, ether, and chloroform. It is soluble 

 in warm glycerin, such solutions having the power of gelatinizing when 

 cooled. 



