232 SOUECE OP THE CASEIN OF MILK. 



The occurrence of casein under the form of kiestine in the urine, in 

 quantity increasing as gestation advances, indicates therefore that the 

 system is assuming a propensity for the generation of this substance from 

 its albumenoid compounds ; and since, in cases of starvation, the percent- 

 age of casein in the milk does not seem to be materially affected, we are 

 to attribute its immediate source to the system rather than to the food. 

 In this respect it differs from the oily constituent, butter, the percentage 

 amount of which is instantly affected by variations in the nature and 

 quantity of the food. It would seem, indeed, that, from the same plastic 

 ingredient, albumen, the soft tissues of both mother and infant are fabri- 

 cated, with this difference, that in the latter case the temporary condition 

 of casein is intermediately assumed. We have already remarked on the 

 identity of constitution of albumen, casein, and fibrin, so far as their car- 

 bon, hydrogen, nitrogen, and oxygen are concerned ; and, indeed, these 

 compounds differ far less in their physical characters from one another 

 than albumen in its coagulated and uncoagulated state ; yet that differ- 

 ence in physical quality may be readily brought about by so trifling an 

 agency as rise of temperature through only a few degrees, and is proba- 

 bly dependent upon the different allotropic forms which the carbon con- 

 stituent is prone to assume. Giving due weight to these various consid- 

 erations, we shall find reason to conclude that this constituent of the 

 milk, the casein, is directly derived from the system, which can manufac- 

 ture it at a rate of about 30 grains per hour, this being about one half 

 the quantity of fibrin generated in the same period of time for the sup- 

 port of the muscular tissues. Chemically, the transition from albumen 

 to casein is not to be regarded either as an ascending or declining meta- 

 morphosis, but only as the temporary assumption of a state of passage 

 onward to the condition of fibrin. 



With respect to the constitution of casein there is considerable doubt. 

 Complex na- The substance commonly passing under this title seems to 

 ture of casein. cons i s t O f a t least two different bodies ; at all events, it may 

 be separated into two parts, one containing sulphur, and the other not ; 

 moreover, if to milk, which has been perfectly freed from butter, there be 

 added dilute hydrochloric acid, the ordinary precipitate is yielded, but 

 there still remains in solution an analogous body, which does not precipi- 

 tate until the mixture is boiled. In milk, though much of the casein is 

 held in solution, much also exists in the coagulated state, forming the 

 wall of the milk globules. Its existence under this membranous form 

 may be demonstrated by the action of acetic acid on milk globules un- 

 der the microscope, and also by shaking new milk with ether, which pro- 

 duces very little change ; whereas, if the milk were only an emulsion, the 

 ether should take up the fat and hold it in solution. Now, on the addi- 

 tion of potash or its carbonate to milk before the action of ether, those 



