570 THE SUBSTANCE OF THE STOMACH CHANGES 



the custom both of preserving the natural contents of the stomach — and 

 of generally throwing away the bag after being once salted, dried, and 

 extracted. The gastric juice which exudes from the interior surface of 

 the stomachs of all animals is known to curdle milk readily, and, there- 

 fore, it was natural to ascribe the action of rennet to the presence of this 

 substance, and to infer that, oeing once extracted, it was in vain to ex- 

 pect much advantage from salting and infusing the membrane a second 

 time. But the three facts — 



a. That in most places it is customary to wash the interior of the 

 stomach before salting it, and thus to remove the greater part of the gas- 

 tric juice it may contain ; 



6. That besides, in many places, the lags are laid up in brine for 

 weeks and months, and are then drained out of this brine before they are 

 dried — by which any gastric juice remaining must be almost entirely re- 

 moved, — and 



c. That after being (hied and steeped once for the preparation of ren- 

 net, experience has proved that they may again be salted and used over 

 again ; 



— these three facts, I think, shew that the efficacy of rennet does not de- 

 pend upon any thing originally contained in the stomach, hut upon 

 something derived from the substance of the stomach itself 



Now when considering the properties of milk-sugar and-of lactic acid, 

 1 have stated that if a piece of the fresli membrane of the stomach or in- 

 testine, or even of the bladder of an animal, be exposed to the air for a 

 few days, and be then immersed into a solution of milk-sugar, it will 

 gradually transform the sugar into lactic acid. In milk this membrane 

 would produce a similar eftect, aiding and hastening the natural souring 

 and curdling effect of the casein. By exposure to the air, the surface of 

 the membrane has undergone such a degree of change or decomposition, 

 as enables it to induce the elements of the sugar to alter their mutual 

 arrangement, and to unite together in such a way as to form lactic acid. 



If the moist membrane be exposed for a longer time to the air this 

 change of its surface will penetrate deeper, and it will become more ef- 

 fective in inducing the transformation of the sugar into lactic acid. But, 

 at the same time, a portion of its surface may run into a state of putre- 

 faction, and besides accpiiring a disagreeable odour may become capable 

 also of bringing on fermentation and putrefactive decay in the solutions 

 upon which it may be made to act. It is not expedient, therefore, to at- 

 tempt to heighten the transforming effect of animal membranes by 

 exposing them for a greater length of !.me to the air in a moist and fresh 

 state. 



But if the membrane be salted, au.l thus preserved from the rapid 

 action of the air, it will be protected from putrefaction in a great degree, 

 while, at the same time, it will undergo that gradual change upon its 

 surface to which its power of transforming solutions of sugar is ascribed. 

 And this change will be materially hastened and increased and made to 

 penetrate deeper, if the salted membrane be subsequently dried slowly 

 in the air by a gentle heat, and be^afterwards kept for a length of time 

 where the air has more or less ready access to it. Such is the mode of 

 treatment to whicli the calf's stomach is subjected for the preparation of 

 rennet, and it is an important practical observation that the membrane 



