82 Journal of Agriculture. [lo Feb., 1909. 



of the ingredients ; such milk tends to check further secretion and_ may 

 induce inflammatory trouble through irritation. The secretion of milk is 

 not under the control of the will but can be affected by the physical and 

 mental state of the animal. Thus the quantity will diminish through 

 sexual excitement, fatigue, insufficient food or water, &c. The secretion 

 of milk becomes very active when the calf is sucking, and even during 

 the act of milking. In consequence the milk gained by the calf or milker 

 may be more than that simply stored in the ducts and reservoirs. 



Physical Characters of Cow's Milk. 



The average specific gravity of cow's milk at 60 deg. F. is about 1,032, 

 that is to say, 1,000 volumes of milk would weigh as much as 1,032 

 volumes of water at the temperature specified. As the fat in milk has a 

 specific gravitv lower even than water it follows that the specific gravity of 

 separated milk is higher than that of whole milk. The white colour of 

 milk is due partly to the fat being' in a state of emulsion and partly to the 

 caseinogen of the milk being to a slight degree also in a state of suspen- 

 sion. The yellowish tinge is due to a pigment associated with the fat. 

 The sediment present in uncontaminated cow's milk is very slight in 

 amount and is composed of fibrin threads which have formed in the milk 

 itself and epithelial cells shed by the lining membrane of the ducts in the 

 udder and teat. During the colostral period (lasting a few days after 

 parturition) milk contains cellular elements — colostral corpuscles — which 

 represent the debris of incompletely broken-down gland cells. Fresh milk 

 is slightly acid to some indicators owing to the presence of phosphates and 

 caseinogen. 



Constituents of Milk. 



I. Caseinogen. This is a complex protein containing phosphorus. It is 

 an acid and is insoluble in water. In milk, however, it occurs as a salt 

 of lime which is soluble. When acid is added to milk, or if the milk 

 develops lactic acid on standing, a clot is formed which is due to the added 

 acid seizing upon the lime and turning the insoluble caseinogen out of 

 combination. When milk is subjected to the action of a protein-splitting 

 ferment such as rennin, trypsin, &c., a totally different form of clotting 

 occurs. The caseinogen is altered chemically being transformed into casein 

 and cannot be restored to its original state. Caseinogen in solution as salt 

 is not precipitated by boiling but it forms a tough skin on the surface. 

 The whole of the caseinogen in milk is not in solution ; part is in sus- 

 pension unattached to lime and thus the white colour of separated milk 

 is produced. Caseinogen is soluble in alkalies and in strong excess of 

 acids, but it is insoluble in weak acids. 



2. Lactalbumen. This is a true albumen being coagulated by heat. It 

 differs only slightly from the albumen in blood. In colostral milk the 

 content of lactalbumen is very high and the milk will, if heated, torm a firm 

 clot. 



3. Fibrinogen. A very small amount of fibrinogen is present which 

 undergoes spontaneous transformation into fine gelatinous threads of fibrin. 



4. Fat. Milk contains a mixture of fats, each being composed of a 

 fatty acid and glycerine. The preponderating fatty acids are oleic, palmitic, 

 myristic, and butvric. The fat is held in suspension in the form of 

 microscopic globules each surrounded by a jacket of precipitated casein- 

 ogen. When milk stands the globules rise to the surface as cream. This 

 action, as is well known, can be accelerated by centrifuging. 



