i MILK 13 



a mixture of gases having the following average composition 



is found : 



CO 2 . 59 '63 per cent. 



O " 13-17 



Other gases 27 '20 



100-00 per cent. 



Finally, certain foreign substances can find their way by 

 accident into the milk, as, for example, from the food or from 

 medicine which the animals have taken ; these cases, though, 

 are quite exceptional. Of those extraneous bodies which are 

 constantly found in milk the first place is taken by the white 

 blood corpuscles, or leucocytes, which pass directly from the 

 blood into the milk through the thin walls of the alveoli. The 

 presence of leucocytes and their importance will be discussed 

 later in the section dealing with the differences between un- 

 heated and heated milk. In addition to the leucocytes, milk 

 also contains other cells and fragments of cells which have 

 arisen from the alveolar epithelium of the mammary gland. 

 Epidermis cells, which have come from the outside of the 

 udder and from the milker's hands, are found almost without 

 exception in milk. Further, in every sample of milk there 

 are found considerable, although varying, numbers of micro- 

 organisms; they are chiefly bacteria, but moulds and yeasts 

 are also present. The solid matter which accidentally finds 

 its way into milk is composed of small, light particles, which 

 are blown about in the stall and so fall into the pail during 

 milking. All these substances comprise the dirt of rnilk, and 

 the amount of this varies according to the degree of cleanliness 

 in the stall, and particularly to the precautions that are observed 

 during milking. 



If the milk is mechanically separated by means of centri- 

 fugal force, these solid impurities, along with some casein and 

 quantities of white blood corpuscles, cells and cell fragments, 

 collect as a grey sticky mass on the sides of the separator 

 drum. This mass, the so-called separator slime or mud, has an 

 analysis somewhat as in the table on page 14. 1 



If this slime is examined microscopically it is seen that along 

 with the mass of bacteria, white blood corpuscles, and other 



1 Chr. Barthel : " Sur la composition de la boue de centrifuge," Retue Gn. 

 du Lait, Vol. I, p. 193. 



