1292 PHYSIOLOGY 



The calcium caseinogenate forms only opalescent solutions. Apparently 

 the compound is dissociated by water with the formation of caseinogen acid 

 which is in a state of partial solution as swollen-up aggregates. It is 

 impossible therefore to filter calcium caseinogenate through a clay cell. It 

 is mainly in this form that caseinogen is contained in milk, hence the 

 opalescent appearance of the milk plasma. When calcium caseinogenate 

 solution is 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 

 modification 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 lactoglobulin. 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 

 degree of acidity 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 

 spontaneous 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 <>!' the vouijo- animal of the same species which wore made by 



