VARIATION IN COMPOSITION OF MILK 13 



During lactation the alveoli secrete milk in the intervals of 

 suckling so that the milk accumulates both in the alveoli and in 

 the ducts, which, in some animals, are provided with sacs and 

 dilatations which serve as reservoirs. The later drawn portions 

 of milk contain more solids, in proportion, than that which is first 

 drawn. 



The exact source of the various constituents comprising fully- 

 formed milk is not known. The probabilities are that the casein 

 is produced by a molecular change in the composition of the serum 

 albumin or globulin, which is supplied to the cells from the blood or 

 lymph. The fat may be formed from proteid, or even carbo- 

 hydrates, present in the blood, or it may be taken up directly 

 from fat which has been formed elsewhere. There is evidence to 

 show that the sugar is derived from dextrose. 



Variation in the composition of milk. — It is obvious that 

 whilst we know generally speaking the standard quality of milk, such 

 standard is not strictly applicable to all kinds of milks. We must 

 bear in mind that in nature, and quite apart from adulteration, we 

 have constantly to deal with considerable variations from the 

 normal standard. These variations affect the constitution of the 

 milk rather as regards its percentage composition than as regards 

 its constituents. The chief causes for natural variation of milk 

 are: — 



(i) The race of cow. — The milk of different herds varies within 

 comparatively wide limits. Probably the best all-round good milk 

 is obtained from dairy Shorthorns or their cross. Jersey and 

 Guernsey cattle yield a milk having large fat globules which readily 

 rise to the surface. The characteristics of their milk are the high 

 percentage of fat and the richness of colour combined with a 

 liberal quantity. Kerrys and Dexter Kerrys yield a milk approach- 

 ing in standard that of the Shorthorn, the latter of the two how- 

 ever being as a rule richer in fat. The milk of Ayrshire cattle 

 contains a high percentage of casein, but is low in fat. The globules 

 are of small average size and the milk rises slowly and has com- 

 paratively little colour. In Devons the quality is good but the 

 quantity small, while the Holstein, on the other hand, gives a large 

 quantity of milk of comparatively poor quality. In the emulsion 

 of milk from Jersey cows the corpuscles are much larger than in 

 the Holstein, while the Holstein emulsion, when broken down, is 

 much more easily restored ; that is, it emulsifies better and resists a 

 destruction of its emulsion, such as occurs when cream is produced, 

 more than do the milks of Jerseys and Guernseys. The emulsion 



