It was formerly the custom among German writers to describe a stage of normal digestion which was characterised by the production of lactic acid. The experiments upon which this view was founded appear, however, to have been falsified by the accidental introduction of lactic acid with the bread of the test meal, and consequently the elaborate theories that were formulated to explain the development of the organic acid have ceased to be either useful or interesting. Boas was the first to call attention to the existence of an excess of lactic acid in stomachs affected by cancer, and subsequent investigations have confirmed his statement upon this point. Like the absence of free hydrochloric acid, the presence of lactic acid is by no means invariable, and it is interesting to observe that in those exceptional cases where the free mineral acid is present the organic acid is usually absent. Among 109 cases collected by Schiff in which lactic acid was found during the period of digestion, the stomach was affected with carcinoma in ninety-two, or 84.4 per cent.; while among 268 cases of malignant disease of the organ, lactic acid was shown to exist in 197, or 73.5 per cent. Strauss noted its presence in 91 per cent., Bosenheim in 78 per cent., Lindner and Kuttner in 60 per cent., Hemmeter in 82 per cent., and Osier and McCrae in 75.3 per cent, of the cases they examined. According to our experience, it is present in about 91 per cent, of the cases where the pylorus is the seat of the disease, and in 57 per cent, of those in which the growth affects the cardia or a circumscribed area of the organ.

The conditions which bring about an excessive formation of lactic acid in the stomach are: (1) Stagnation of the food ; (2) a deficiency of hydrochloric acid; (3) a diminished power of digestion and absorption of proteids. In cases of gastric cancer, and especially in those where there is stenosis of the pylorus, all these conditions are present, and consequently malignant disease is more often associated with lactic acid fermentation than any other affection. It is important to remember, however, that the same requirements are sometimes fulfilled by other lesions than carcinoma of the stomach, such, for example, as benign stenosis of the pylorus accompanied by atrophic gastritis, or partial occlusion of the lower duodenum by a malignant growth or the pressure of a tumour. While, therefore, the discovery of lactic acid in the gastric contents cannot be considered as pathognomonic of carcinoma of the stomach, its existence along with other symptoms and signs of the complaint must be regarded as confirmatory evidence of the disease.

Many elaborate processes have been invented by which lactic acid may be detected and its quantity estimated, but their value as an aid to diagnosis is extremely doubtful. All that is required is some method, easy of application, by means of which the substance can be recognised when present in excess. This simple qualification is amply fulfilled by the test proposed by Ueffelmann, which consists in mixing 10 c.c. of a 4 per cent, solution of carbolic acid with 20 c.c. of water containing one drop of a strong solution of perchloride of iron. The mixture presents an amethyst-blue colour, which changes to canary-yellow on the adddition of lactic acid. The delicacy of the reaction is destroyed by the presence of free hydrochloric acid, and a somewhat similar colouration is produced by glucose, phosphates, and alcohol. If any doubt exists as to the result of the test, a small quantity of the filtered gastric fluid should be thoroughly shaken with ten times its volume of pure ether, and allowed to stand until the fluids separate from one another. The ethereal portion of the mixture is then siphoned off and gently evaporated, and the residue is dissolved in distilled water and tested for lactic acid as in the previous case.

In rare instances the contents of the stomach extracted one hour after a test meal are found to be neutral or alkaline, and to contain neither lactic nor hydrochloric acid. This absence of acidity neither supports nor negatives the existence of carcinoma.

Secretion of Pepsin and Rennet. -The digestive power of the gastric juice is always greatly impaired by a deficiency of hydrochloric acid, although it is now accepted that, so long as the stomach is able to absorb peptones, a combination of pepsin and lactic acid is sufficient for the digestion of proteids. In every case the filtered contents of the stomach, when acidulated with hydrochloric acid, exhibit marked digestive properties, and if lavage is performed with a 0.2 per cent, solution of the mineral acid the resultant fluid will readily dissolve egg albumin at the temperature of the body. Although the results have no particular value from a clinical standpoint, the quantity of pepsin secreted by the stomach may be determined by the following method, devised by Hammerslag: 10 c.c. of a 1 per cent, solution of egg albumin, containing free hydrochloric acid to the amount of 0.4 per cent., arc placed in an Esbach albuminometer, and to them are added 5 c.c. of the gastric filtrate to be examined. A second albuminometer is filled with 10 c.c. of the standard solution of egg albumin and hydrochloric acid, and both tubes are allowed to remain in an incubator at a temperature of 98° F. for an hour. At the end of this time the amount of albumin in each is determined by the Esbach reagent, when the activity of the pepsin in the gastric filtrate may be roughly gauged by observing the difference in the amount of precipitated albumin in the two tubes.

According to Boas, the rennet ferment usually persists after the disappearance of hydrochloric acid, but this fact has little clinical significance. Its existence in the gastric contents may be determined by neutralising 5 c.c. of the latter with carbonate of sodium and adding an equal quantity of milk. After standing for fifteen minutes in a warm place a firm coagulum of casein will be found to have formed if rennet was present in the gastric fluid.