Another important function of gastric juice is that of curdling milk, due to the presence in the secretion of a peculiar enzyme known as rennin. The latter ferment acts upon the casein of milk,--the chief proteid constituent,--transforming it into a related substance commonly called paracasein. This then reacts with the calcium salts present in milk, forming an insoluble curd or calcium compound. From this point on, the digestion of milk-casein by gastric juice is the same as that of any other solid proteid, it being gradually transformed by the pepsin-acid into soluble cleavage products. Why gastric juice should be provided with this special enzyme, capable of acting solely on the casein of milk, can only be conjectured, but we may assume that it has to do with the economical use of this important food. As the sole nutriment of the young, milk occupies a peculiar position as a foodstuff, and being a liquid, its proteid constituent might easily escape complete digestion were it to pass on too hastily through the gastro-intestinal tract. Experiment has shown that when liquid food alone is taken into the stomach it is pushed forward into the small intestine in a comparatively short time. Curdled as it is by rennin, however, casein must stay for a longer period in the stomach, like any other solid food, and its partial digestion by gastric juice thereby made certain. For the reasons above stated, it is apparent why milk should not be treated as a drink in our daily diet. Remembering that when milk reaches the stomach it is converted into a solid clot or curd, there is obvious reason for sipping it, instead of taking it by the glassful, thereby favoring the formation of small, individual clots instead of one large curd, and thus facilitating instead of retarding digestion.