12 40 PHYSIOLOGY 



boiled it forms a pellicle on the surface in the same way as milk does. On 

 treating the caseinogen with rennet ferment it is converted into a modifica- 

 tion known as paracasein, which in the presence of lime salts is thrown out 

 as insoluble casein. To this process is due the clotting of whole milk by 

 rennet, which is made use of in the preparation of cheese, the curd consisting 

 of a network of casein enclosing fat globules in its meshes. On allowing 

 the clot to stand it shrinks, pressing out a milk-serum. 



From the milk-serum or whey may be obtained two other proteins, 

 known as lactalbumen and Uctoglobulin. These resemble very nearly the 

 albumen and globulin of blood-serum. They are coagulated on heating. 

 According to some authors a third protein is present in the whey, to which 

 the name whey-protein has been given, and which is supposed to be split off 

 from the caseinogen under the action of the rennet ferment. 



Milk can be boiled without* undergoing any coagulation. If it be 

 allowed to stand and become sour by the formation of lactic acid, at a certain 

 period boiling the milk causes its complete coagulation. Later on the acid 

 produced is sufficient in itself to precipitate the caseinogen. Both these 

 processes, namely, coagulation of half-sour milk by heating, and spon- 

 taneous clotting of milk by the production of acid, are made use of in 

 different countries for the manufacture of cheese. 



MILK SUGAR. The sugar of milk, or lactose, is most easily obtained 

 from whey, which, after separation of the clot, is boiled to precipitate the 

 remaining proteins. On filtering and evaporating slowly, the milk sugar 

 crystallises out. Lactose is a disaccharide and has the formula C 12 H 22 O n . 

 It is only known to occur in milk. It may be found in the urine of nursing 

 women when the breasts are not kept empty, so that there is reabsorption 

 of the lactose formed in the mammary glands. It is unaltered by ordinary 

 yeast, so that the yeast test is the best means of distinguishing lactose from 

 dextrose in the urine. It gives the ordinary tests for reducing sugar. The 

 salts of milk include insoluble salts, soluble calcium salts, sodium and 

 potassium, phosphates and chlorides. 



Mere enumeration of the constituents of milk presents but little interest 

 unless we realise how closely the composition of this fluid is adapted to the 

 needs of the growing animal. In the first place, we find a proportionality 

 between the total solids of the milk and the rate at which the young animal 

 grows. It must be remembered that the milk taken by the animal serves 

 only in part for the production of energy in its body, a great proportion of 

 it being required for the building up of new tissue. In no respect is this 

 correspondence seen better than in the comparative analyses of the ash of 

 milk and of the young animal of the same species which were made by 

 Bunge. The following Table shows the composition of the ash of a 

 rabbit fourteen days old, of the milk which it was receiving from its 

 mother, of the ash of rabbit's blood and blood-serum. Nothing could be 

 more striking than the marvellous way in which the cells of the mammary 

 gland have picked out from the salts of the circulating plasma exactly those 

 salts which are needed for the growing animal and in the same proportion : 



