In cases of indigestion from various causes the curds may remain undissolved in the stomach, eventually irritating it and causing vomiting, or they may pass along the intestine and be voided unaltered in the stools. This occurrence is more frequently noticed in young infants than in adults.

Milk which disagrees in the stomach forms large and somewhat tough coagulae of casein, which are with difficulty dissolved by the gastric juice. Many substances may be added to milk which by their mechanical presence will prevent this occurrence, and hence favour the action of the gastric juice.. Excessive acidity of the stomach due either to hypersecretion of hydrochloric acid or the presence of organic acids, especially lactic, derived from the food or fermentation processes, modifies the digestion of milk; the lactose is altered into lactic acid and the casein is promptly coagulated. The alkaline salts of milk are split up and phosphoric acid is liberated, and complicated fermentative changes ensue which are as yet imperfectly understood. All this causes more or less gastro-enteric irritation, resulting in diarrhoea.

When milk or cheese remains too long in the stomach or intestine a further fermentation is excited accompanied by a neutral or alkaline reaction, and which results in the final production of butyric acid, and sometimes of other substances, such as leucin, tyrosin, and ammonia. But while free hydrochloric acid exists in the contents of the stomach, the organic acids - lactic, butyric and acetic - which are associated with milk fermentation are unlikely to develop.

Cow's milk may readily become acid from alteration in the relative amount of potassium biphosphate and the two-thirds phosphate of potassium. The former, if present in excess, hastens coagulation and an acid reaction. When warm the milk may again become alkaline (Soxhlet, Heintz).

These facts explain the frequent necessity of using antacids, such as lime water or sodium bicarbonate, with infant milk food. Mucous 7 fermentation of the proteids makes the milk slimy, and it becomes stringy on boiling.

Milk, according to Rubner's experiments, yields more bulky residue in the feces than either eggs or meat, yet its nutritive ingredients are very perfectly absorbed. From 8 to 10 per cent approximately of the solids of milk is eliminated in the stools.