CHAP. VII.] THE CONNECTIVE TISSUES. 253 



soluble in water are removed. The watery extract contains a little 

 alkaline albuminate, but very little coagulable albumin. The frag- 

 ments of tendon are then soaked for some days in very weak solution 

 of baryta or lime, by the action of which the connecting substance is 

 dissolved so that the individual fibrils fall asunder. The insoluble 

 matter is then washed, first in water and afterwards in weak acetic 

 acid, finally again in water. The residue consists almost entirely of 

 the substance of the white fibrils (collagen), mixed however with 

 small quantities of yellow elastic tissue and cell nuclei. 



When placed in very diluted acids and alkalies, the fibrils swell 

 up and become transparent, their original appearance being restored 

 if the acid is exactly neutralized. When digested in very dilute 

 acetic acid at ordinary temperatures for some days, the fibres 

 gradually dissolve, yielding a solution which contains gelatin, and 

 also a little acid-albumin, produced by the action of the acid upon the 

 residual matter of connective tissue cells. 



When the white fibrils are subjected to long-con- 

 Gelatin. ,. , , .,. . ,, ,. J & ,, ,, 



turned boiling in water at the ordinary pressure of the 



atmosphere, or, still better, to the action of water heated under pres- 

 sure (as in Papin's digester), they dissolve, and the solution is found 

 to contain a substance termed Gelatin. The same body is produced at 

 the temperature of the animal body by the prolonged action of very 

 diluted acids on collagen. 



Prepara- It is most convenient to employ commercial gelatin 



Geiatto PUre for the P re P aration of tne P ure substance. The finest 

 commercial gelatin is allowed to soak for some days in 

 large quantities of distilled water, which is frequently changed; in 

 this way the soluble salts of the gelatin diffuse out. The swollen 

 gelatin is now dissolved in distilled water by the heat of a water 

 bath; after allowing insoluble matters to subside the solution is filtered, 

 with the aid of a hot-water funnel, directly into 90 per cent, alcohol. 

 The gelatin separates in the form of white, thready masses, which are 

 collected, reduced to a fine state of division by cutting, and allowed 

 to dry, first of all in the air, and then in a water oven. Gelatin thus 

 prepared contains about 0'6 per cent, of ash 1 . 



Gelatin is a body which is insoluble in cold water ; the sole 

 action of cold water is to cause it to swell up. On adding boiling 

 water to the swollen solid, it dissolves with readiness, and a clear 

 limpid solution is obtained, which when it is cooled sets as a more or 

 less firm jelly gelatinizes. This property is possessed by solutions 

 which contain only 1 per cent, of the solid substance. 



The power of gelatinizing is gradually lost when solutions of 

 gelatin are subjected to prolonged heating, and instantly lost when 

 they are heated to 140 in sealed tubes. 



1 Hofmeister: "Ueber die chemische Structur des Collagens." Zeitschr. f. phys. 

 Chem. Vol. n. (1878), p. 315. 



