636 VETERINARY PHYSIOLOGY 



has already been shown (p. 372) that the percentage of 

 proteins present varies with the rate of formation of new tissue. 

 The relationship between the salts of the milk and those con- 

 tained in the tissues of the growing animal is shown by the 

 following table given by Bunge. The percentage composi- 

 tion of the chief Id organic constituents of the tissues of a 

 young rabbit, of the milk it was receiving, and of the serum 

 of the mother's blood are compared :— 



This close adaptation of the composition of the milk to 

 the needs of growth aftbrds an explanation for the difference 

 in percentage composition of the milk of different species. 

 It also accounts for the well-recognised fact that, after wean- 

 ing, especially if this takes place too early, the rate of gain 

 of weight per day suffers a marked decrease, since, in 

 practice, no combination of food-stuffs can yield the perfect 

 proportion of the necessary materials which is present in 

 milk. 



(2) Constituents — (i.) Protein. — The chief protein of milk 

 is casein, a phospho-protein. It exists in milk in the form 

 of a calcium salt. Casein contains all the amino-acids, 

 except one, necessary for building up the various proteins of 

 the young animal. The one absent — glycine — can be easily 

 formed in the body from other amino-acids. The other 

 proteins present are an albumin and a globulin, which closely 

 resemble those found in the blood. 



(ii.) Fat, — Milk fat consists of olein, palmitin, and stearin 

 to the extent of nearly 90 per cent. The remaining 10 

 per cent, consists of fats of lower molecular weight. It is 

 the latter that gives the characteristic flavour to butter. 

 Lecithin and cholesterin are also present. The phospho- 



