CASEIN. 385 



and indeed this is the case if a little caustic or carbonated alkali be 

 added to the milk before it is shaken with ether. Mitscherlich, by 

 this beautiful experiment, has removed all doubt regarding the 

 existence of such a membrane. I have, however, observed the fol- 

 lowing facts : on placing under the microscope milk shaken with ether 

 but to which no potash has been added, the surface of the milk- 

 globules appears of diminished transparency, opaque, and fissured; 

 in short, the wall presents the appearance of being coagulated. In 

 place of potash I have used phosphate of soda and sulphate of soda ; 

 milk, treated with the former, yielded almost all its fat to ether, but 

 did not become so clear as when treated with potash ; under the 

 microscope the aqueous fluid exhibited only a few fat-globules, 

 which were no longer round but corrugated, of a caudate form, &c. 

 Sulphate of soda has the property of causing the capsules of the 

 milk-globules to burst, after which the fat can be extracted from 

 the milk by ether ; the watery fluid, however, remains very turbid, 

 but no longer exhibits under the microscope either milk-globules, 

 or shreds of destroyed capsules, but only extremely minute, scarcely 

 isolable, molecular granules, which are unquestionably the frag- 

 ments of the destroyed capsules, and do not consist of finely com- 

 minated fat ; for, on the addition of a little potash, they not only 

 do not disappear under the microscope, but the fluid which had 

 previously retained its milky colour becomes perfectly clear and 

 limpid. Hence we perceive that our ordinary casein not only 

 contains the protein-compound dissolved in the milk, but likewise 

 another, which forms the capsule of the milk-corpuscles, so that 

 we thus also have a microscopico-mechanical proof of the compo- 

 site nature of ordinary casein. 



It was formerly supposed that casein existed in other animal 

 fluids and solid parts, and indeed it was regarded as a normal con- 

 stituent of the blood. In our consideration of the means by which 

 casein may be recognised with certainty, we have, however, shown 

 that no reliance can be placed on statements of this nature. Hence 

 we can attach no weight to the assertions that casein occurs in the 

 urine or in effusions within the peritoneum, the pleura, or the arach- 

 noid, and the cases where, in consequence of metastasis of the 

 milk, casein actually occurs in the urine or other fluids, require no 

 further mention. The same remark holds good in reference to the 

 supposed occurrence of casein in the saliva, in pus, tubercles, and 

 other morbid products. 



Origin. In our entire ignorance of the true chemical constitu- 

 tion of casein, we cannot resort to any experiment to elucidate its 



2 c 



