326 Dr. R. D. Thomson on the Digestion of 



food alone. In general the species of food selected was por- 

 ridge, or a mixture of oatmeal and water well-boiled. In such 

 experiments as I have seen detailed, in reference to the free 

 acid of the stomach, there appears to have been too little at- 

 tention paid to the possible results which might arise from a 

 difference in the nature of the food. Thus the products of 

 the digestion of starch we should naturally expect to be dif- 

 ferent from those of the digestion of animal fibrin or albumen, 

 since we know that lactic acid can be pi'oduced by a modifi- 

 cation of starch, although the same change does not attend 

 the decomposition of the animal substances enumerated. For 

 example, in the preparation of starch an acid liquor collects 

 on the surface of the vessels in which the starch is digested, 

 and in the formation of soisoans from oat-husks and water b}' 

 steeping, an acid liquor is developed, — in both instances a con- 

 sequence of the production of lactic acid at the expense of 

 starch ; while in the German dish, saurkraut, the same acid 

 is generated by an analogous action. I do not at present in- 

 tend to discuss the nature of the acid which presents itself in 

 the stomach during the digestion of animal food, but simply 

 to detail the results of a series of experiments upon the changes 

 produced on vegetable food and starch during digestion. I 

 shall merely content myself with stating, that I have never 

 found a volatile acid in the stomachs of animals which were 

 digesting animal food alone, while in these cases I have inva- 

 riably found an acid to be present which was fixed at the tem- 

 perature of 212°, and even considerably higher. 



The nature of the Acid developed duri7ig the digestion of Starch. 

 — It does not appear an invariable rule that an acid reaction 

 should always characterize the liquid present in the stomach 

 during digestion, as appears from the following experiment. 

 On the 1 1th of July 1844, the different stomachs of a sheep 

 killed twenty-four hours after partaking of grass, contained 

 between two and three pounds of finely-divided green matter 

 exhibiting a pulpy consistence, but being entirely destitute of 

 either an acid or alkaline reaction. The pulpy masses were 

 tested in each of the stomachs with the same negative result, 

 and they were afterwards mixed together, diluted with distilled 

 water and filtered, but still without producing any effect on 

 litmus paper. Experience teaches us, that to determine the 

 presence of a volatile acid in the stomach, great care must be 

 taken in reference to the mode in which the distillation is per- 

 formed, since if the heat of an open fire is applied to the 

 retort, an indication of the presence of hydrochloric acid will 

 be found in the liquid contained in the receiver by the addi- 

 tion of nitrate of silver. The following experiment affbrd.s 



