THE ACID OF THE GASTRIC JUICE. 351 
much saliva, as the total solids and amount of acid are much less than those 
usually found. Analyses II. and III. are of dog's saliva. Analysis II. gives 
the mean of ten determinations, in the case of a dog in which all the salivary 
ducts had heen ligatured. Analysis III. gives the mean of three, in the case of 
a dog with normal salivary glands. Analysis IV. is that of the gastric juice 
of a sheep. 
The acid of the gastric juice. — The acid of the gastric juice 
has probably green vise to more discussion than any other subject in 
physiological chemistry. The principal points for consideration are — 
(a) the nature of the acid ; (b) the seat of formation and the mode of 
origin of the acid : (c) the function of the acid. 
The nature of the acid. — Before discussing this question in detail, 
it may be well to state clearly the present state of opinion on the 
subject. 
It has been demonstrated that hydrochloric acid is the principal acid 
of the gastric juice, and that in the purer samples free from food it is 
always present, and is almost exclusively the only acid present ; while in 
gastric juice mixed with food, especially with carbohydrate food, it may 
be, and often undoubtedly is, accompanied by lactic acid. C. Schmidt, 
from a large numl >er of painstaking and laborious analyses, concluded 
that the pure gastric juice of carnivora, obtained after a fast of eighteen 
to twenty hours, contains only hydrochloric acid, and no trace of lactic or 
acetic acids ; while the gastric juice of herbivora contains, besides hydro- 
chloric acid, small quantities of lactic acid, but this is even then probably 
from remnants of carbohydrate food. 1 
Prout, 2 in 1824, first showed that gastric juice contains free hydro- 
chloric acid by the following method : — 
The contents of a stomach were mixed up with water, and, after the 
mixture had settled, the clear part was removed by decantation. This 
was divided into three equal portions, a, b, and c. 
(a) The first portion was evaporated to dryness, incinerated, and the 
total amount of chlorine in the ash determined by weighing, as silver 
chloride. 
(b) The second portion was first made alkaline by the addition of 
potash, then evaporated to dryness, incinerated, and the total chlorine 
determined as before. 
(c) In the third portion, the total acidity was determined by titration 
against standard alkali, and reckoned as hydrochloric acid. 
In portion (a) all the free acid is driven off as well as any which may 
be combined with volatile or decomposable bases (such as ammonium 
chloride) ; in portion (b) all the chlorine remains, that which was either 
free or combined with ammonia becoming converted into non-volatile 
potassium chloride ; therefore the difference of (b) and (a) gives the free 
hydrochloric acid, plus any volatile chlorides which may be present. In 
(c) all the acid is estimated as hydrochloric acid, and by subtracting 
this from the difference of (b) and (a) the amount present as volatile 
chlorides is obtained. 
Prout also showed that when gastric juice is distilled, towards the 
1 The stomach of the herbivora retains food for a much longer period than that of 
carnivora. Traces of food are usually found in the stomach of the sheep even thirty-six 
hours after a meal. See CI. Bernard, " Lecons de physiol. exper." 1856, tome ii. p. 
389. 
2 Phil. Trans., London, 1824, part i. p. 45. 
