343 
seems also to be the only native albumin which is attacked by 
erepsin (see Cohnheim (3)), and on account of its ease of hydroly- 
sis it probably plays a special part in metabolism. In this con- 
nection Tunnicliffe (3a) has shown that when total digestibility is 
considered human milk is much more digestible than any of its 
substitutes. 
Among the various changes brought about in the composition of 
milk through the action of the digestive ferments the most typical 
and characteristic is the rennin coagulation. Exclusive of the 
mineral matter the caseinogen is the only constituent of the milk 
involved in this change. 
It has long been known that fresh milk coagulates in the stomach 
of higher animals, and that an aqueous extract of the inner lining 
of the stomach of the calf, when added to fresh milk, causes it to 
curdle, whereby it clots or sets in the form of a solid curd. Since 
early times this fact has been turned to practical account in the 
making of cheese. The earlier explanations of the rennin coagu- 
lation of milk were based on an observation by Fremy (4) to the 
effect that rennet or the mucous lining of the calf s stomach has the 
power of converting milk sugar into lactic acid. According to 
Liebig (5), therefore, rennin curdles milk for the reason that it acts 
upon the milk sugar, converting it into lactic acid. The latter then 
neutralizes the alkali of the milk which holds the caseinogen in 
solution, thereby precipitating this substance as the curd. 
Soxhlet (6) also saw in the curdling of milk by rennin an analogy 
to the coagulation of milk by acids. According to him the former 
process took place much more rapidly than the latter. He held 
with Liebig that the rennin converted the sugar of milk into lactic 
acid and that this in turn converted the alkaline phosphate existing 
in the milk into an acid phosphate, which in turn precipitated the 
casein. Hallier (7) explained the rennin coagulation of milk as due 
to the presence of micro-organisms in the stomach of the calf. The 
most important of the earlier observations on the curdling of milk 
by rennin was that made by Heintz (8), who showed that contrary 
to previous teachings on the subject the aqueous extract of the 
mucosa of the calf s stomach has the power of curdling milk in both 
acid and alkaline solutions. To Hammarsten (9) and Schmidt (10), 
however, belong the credit of first showing that the rennin curdling 
of milk is accomplished by means of a soluble ferment to winch they 
gave the name of “labferment ” or “chymosin.” This is the fer- 
ment which in English is called rennin, formerly rennet. Ham- 
marsten succeeded in showing that the curdling of the milk by 
rennin is independent of the action of lactic acid ; second, that the 
caseinogen (casein) of milk is not in true solution in milk but in 
