154 ORGANISED FLUIDS. 



From the above brief sketch of the constitution of the 

 milk, it will be seen that a very close analogy exists between 

 that fluid and the blood : like it, the milk is made up of two 

 parts, the one solid, the other liquid ; like it too, it con- 

 tains all the elements requisite for nourishment and deve- 

 lopment, it serving for both during a very long period of the 

 life of the human species. 



THE SERUM OF THE MILK. 



The separation of the serum and the globules, accom- 

 plished in an imperfect manner naturally, is more completely 

 effected artificially by filtration, the serum, by means of the 

 ordinary filtering paper, may be obtained transparent and 

 colourless, almost free from globules, these being for the 

 most part retained upon the filter. 



It may however be observed, that the first portions of 

 serum which pass through the paper will be more or less 

 coloured in consequence of their containing a certain number 

 of the smaller globules which had escaped through the in- 

 terstices of the paper ; these coloured and semi-opaque por- 

 tions of serum should be rejected. 



The serum contains dissolved in it the sugar, and the 

 principal portion of the cheese of milk, as well as certain 

 salts, the principal of which are pointed out in the analysis 

 of Haidlen. 



The cheese or caseine is an animal principle, which in its 

 properties approaches closely fibrin : it is precipitated by the 

 mineral, the acetic, and the lactic acids. 



Although the greater portion of the caseine exists in the 

 milk in a fluid condition, M. Quevenne appears to have 

 established the fact, that it is also present in a solid state in 

 the form of globules, these being exceedingly small and re- 

 fracting but slightly the light ; the same globules may also 

 be detected in recently precipitated cheese. (See Plate XV. 



Jig, 5.) 



M. Donne has shown that these cheese globules may be 

 demonstrated by the process of filtration ; thus, the first few 



