Anatowy and Physiology of the Mammary Gland. 95 



to their presence. Possessing a less specific gravity than the fluid 

 in whicli they float, they rise to the surface of milk kept at 

 rest, conslituting the cream. 



Fie 



7. 



The microscopical appearance of 

 the milk globules — the oleaginous 

 portion of the fluid. These bodies 

 vary considerably in size, as is 

 shown by the figure ; some exist- 

 ing as mere points, and others mea- 

 suring about ^uVo-'h of an inch in 

 ^iiameter. They exist in increased 

 or diminished numbers in propor- 

 tion as milk is ricii or poor. Tliey 

 <Io not coalesce in healthy milk, 

 ])eing apparently surrounded by a 

 thin covering of insoluble matter, 

 which keeps them asunder. 



Di\ Carpenter thus writes : " If it (milk) be allowed to stand 

 for some time exposed to the air, a large part of the olea- 

 ginous globules come to the surface, in consequence of their 

 inferior specific gravity, and thus is formed tlie creavi, which 

 includes also a considerable amount of caseine with the sugar 

 and salts of the milk. These may be partly separated by the 

 continued agitation of the cream, as in the process of churning ; 

 this, by rupturing the envelopes of the oil-globules, separates it 

 into butter, formed by their aggregation, and huttermilk, containing 

 the caseine, sugar, &c. A considerable quantity of caseine is 

 still entangled with the oleaginous matter, and this has a tendency 

 to decompose, so as to render the butter rancid. It may be 

 separated Ijy keeping the butter melted at a temperature of 180*^, 

 when it will fall to the bottom, leaving the butter pure, and much 

 less liable to change; an operation which is commonly known 

 as the clarifying of butter. The milk, after the cream has been 

 removed, still contains the greater part of the caseine and sugar. 

 If it be kept long enough a spontaneous change takes place in 

 its composition, an incipient change in the caseine being the 

 cause ot the conversion of the sugar into lactic acid ; and this 

 coagulating the caseine, by precipitating it in small flakes. The 

 same precipitation may be accomplished at any time by the 

 agency of various acids, especially the acetic, which does not act 

 upon albumen ; but caseine cannot be coagulated like albumen 

 by heat alone. 



" Tiie most complete coagulation of caseine is effected by the 

 agency of the dried stomach of the calf, known as rennet ; which 



