CHAP, v.] NUTRITION. 429 



sent, however, we may be content with the following conclusions. 

 1. Fat is actually formed in the animal body, and is not merely 

 stored up from the fat of the food. 2. The carbon elements of 

 .the newly-formed fat may be supplied either from amylaceous 

 food, or from the carbon surplus of proteid food, or from fats 

 taken as food which are not the natural constituents of the body- 

 fat. 3. The fat stored up appears as fat granules or drops de- 

 posited in the protoplasm of certain cells, and the increase of the 

 fat in the cells is accompanied first by a growth, and subsequently 

 by a decay of the protoplasm; but as in the analogous case 

 of glycogen there is no complete evidence to shew whether 

 the fat-granules which appear are simply deposited by the proto- 

 plasm in a more or less mechanical manner, without their forming 

 an integral portion of it, the chief stages of the manufacture 

 of the fat having been gone through elsewhere, or whether they 

 arise from a breaking up, a functional metabolism of the proto- 

 plasm of the fat-cell itself; the latter view is on the whole 

 however the most probable. 



The Mammary Gland. 



Since milk is a secretion, and indeed an excretion, the mam- 

 mary gland ought not to be classed as a metabolic tissue, in the 

 limited meaning we are now attaching to those words. Yet the 

 metabolic phenomena giving rise to the secretion of milk are so 

 marked and distinct, and have so many analogies with the purely 

 metabolic events in adipose tissue, that it will be more convenient 

 to consider the matter here, rather than in any other connection. 



Human milk has a specific gravity of from 1/028 to T034, and 

 when quite fresh possesses a slightly alkaline reaction. It speedily 

 becomes acid, and cow's milk, even when quite fresh, is sometimes 

 slightly acid, the change of reaction taking place during the 

 stagnation of the milk in the mammary ducts. 



The constituents of milk are : 



1. Proteids, viz. casein 1 , and an albumin, agreeing in its gene- 

 ral features with ordinary serum-albumin. The casein may be sepa- 

 rated by curdling with rennet (p. 246) ; it may also be thrown down 

 by the careful addition of acetic acid, but a more complete precipita- 

 tion is effected by first adding to the milk a slight quantity of 

 acetic acid, and then passing through it a stream of carbonic acid. 

 From the nitrate the serum-albumin, which is present in small and 

 variable quantities, may be obtained by coagulation with heat, 

 or by precipitation with potassium ferrocyanide, &c. 



1 Or, if we restiict the -word casein to the substance which appears in a solid 

 form in curdling, or which may be precipitated by acids, an antecedent of casein. 



