lo Feb., 1909.] Elements of Animal Physiology. 



83 



5. Lactose. This is a disaccharide sugar, as already explained in a 

 previous chapter. Lactose does not readily undergo alcholic fermentation 

 but is very readily attacked by various bacilli, becoming transformed into 

 lactic acid — hence the spontaneous souring of milk. 



6. Salts. Chlorides and phosphates of soda, lime, potash, and mag- 

 nesia are present, but the preponderating salt is phosphate of lime as this 

 is needed for the rapid bone growth of the young animal. 



7. Lipoid. Lecithin and cholestrin are present but not in such quan- 

 tities is cow's milk as in human. 



8. Organic acids. These reckoned as citric acid are present in cow's 

 milk to the extent of about 0.25 per cent. 



9. Other substances. Without doubt there are many substances present 

 in small quantities which further research will discover and estimate. There 

 is evidence that milk contains antitoxins which are absorbed best in the 

 colostral period ; also enzymes, the use of which is unknown ; the precursors 

 of hormones, &c. The pigment has already been mentioned. 



We may regard milk as a perfect food for the young of the same 

 species as the milk-secreting animal. It is impossible to alter the milk of 

 any animal to make it suit the requirements of the young of another species. 



Average Percentage Composition of the Milk of Various Animals. 



Water 



Casein- gen 

 Lactalbumen 

 Fat ... 

 Lactose 

 Salts ... 



90.2 

 1-5 



3*1 

 5 



0*2 



Variation in Milk. 



The milk of all mammals varies somewhat with the period of lactation 

 to suit the needs of the young. Variations in food supply affect chiefly 

 the quantity but not the relative proportions of the ingredients. The milk 

 fat can alter slightly in character when certain oily foods are administered. 

 Thus linseed meal gives a butter with a low melting point and bran one with 

 a high melting point. Stagnation of milk in the udder causes a fall in the 

 fat content. The breed of the cow is a most important consideration, 

 witness the high fat content of milk from Jersey cows and the much lower 

 fat content of milk from Ayrshires. In milking the last drawn milk is 

 much richer in fat than the first drawn. This is probably due to a number 

 of factors, one of them being that the larger fat globules meet with more 

 resistance in passing through the small ducts and so come out only towards 

 the end of milking. When the food of a lactating mammal contains 

 highly diffusible and strongly flavoured substances these are apt to appear 

 in the milk after a short interval of time. If the food in question be not 

 repeated the highly flavoured substances will be reabsorbed into the blood 

 and removed from the sy.stem by the kidneys. Thus if a cow has a feed 

 of cabbage or garlic and is milked a few hours afterwards the milk is 

 strongly tainted with undesirable flavouring matter ; but if the milking be 

 carried out immediately after, or twelve hours after the feed, the milk 

 may be free from the objectionable flavour. 



