1386 PHYSIOLOGY 



Milk forms an opaque white fluid with characteristic odour and sweetish 

 taste. Its specific gravity varies between 1028 and 1034. Its reac- 

 tion to litmus is neutral, to lacmoid it reacts alkaline, and to phenol- 

 phthalein, acid. One hundred cubic centimetres of fresh milk, when 

 treated with lacmoid, requires 41 c.c. njW acid for neutralisation. 

 When treated with phenolphthalein the same amount requires 

 19'5 ft/10 alkali for neutralisation. When exposed to the air milk 

 rapidly undergoes changes in consequence of infection by micro- 

 organisms. The most common of these changes is the formation of 

 lactic acid by the bacillus lacticus. In some cases the milk may 

 undergo a species of alcoholic fermentation, as in the formation of 

 kephir, which is made by the fermentation of mares' milk. 



The opaque appearance of milk is due chiefly to the presence of 

 multitudes of fine fatty particles. On allowing the milk to stand 

 the particles rise to the surface, forming cream, and by a mechanical 

 agitation, especially if the milk is slightly sour, they may be caused 

 to run together with the formation of butter. Much discussion has 

 arisen as to the reason why the fat globules do not run together natur- 

 ally. By many authors it has been imagined that they are clothed 

 with a special protein membrane (haptogen membrane) originating from 

 the protoplasm of the cell in which the fat globules were originally 

 formed. It must be remembered that in any protein solution, such as 

 that in which the globules are suspended, the protein tends to aggre- 

 gate, with the formation of a pellicle, at the surface, so that an emulsion 

 once produced in such a fluid will tend to be more or less permanent. 

 There seems therefore no reason to assume the presence of a distinct 

 membrane differing in composition from the proteins present in the 

 surrounding fluid. The fats of milk consist for the greater part of the 

 neutral glycerides, tripalmitin, tristearin, and triolein. In smaller 

 quantities it contains the triglycerides of myristic acid, butyric acid (?), 

 and capronic acid, with traces of caprylic, capric, and lauric acids. 



The milk plasma, the fluid in which the fat globules are suspended, 

 contains various proteins, a carbohydrate, lactose, and inorganic salts, 

 with a small amount of lecithin and nitrogenous extractives. 



THE PROTEINS OF MILK. The chief protein of milk is caseino* 

 gen, belonging to the class of phosphoproteins. Like other bodies of 

 this class it presents distinct acid characteristics, being precipitated 

 by acids and soluble in dilute alkalies. It may be prepared from 

 separated milk by the addition of weak acids. A convenient method 

 is to dilute one litre of milk with ten litres of distilled water and add 

 to the mixture 10 c.c. of glacial acetic acid. The precipitate which is 

 formed rapidly sinks to the bottom and may be washed two or three 

 times by decantation. It may be purified by solution in dilute 

 ammonia and precipitation by acetic acid two or three times. The 



