THE GENERAL COMPOSITION OF MILK 15 



periods after parturition. It is possible that the mammary gland 

 may act to a slight extent as an excretory organ, when there is 

 a surplus of waste substances circulating in the blood stream. 1 

 This condition will be the case in the later months of pregnancy 

 at all events, and probably occurs in the early months. In this 

 case the slight rise in nitrogen may be partly, if not wholly, 

 accounted for. It would appear that no observations upon the 

 metabolism of pregnant cows are available. Investigations upon 

 the metabolism of pregnancy and lactation have been carried out 

 upon women by Mellanby, and upon dogs by Murlin and by Dienes. 

 For further details and additional literature, reference should be 

 made to these authors. Detailed discussion of this subject is 

 outside the scope of the present work. 



Medical literature contains much discussion upon the desira- 

 bility or otherwise of weaning an infant should pregnancy super- 

 vene. The general opinion appears to tend in the direction that 

 weaning is not necessary unless the health of the mother should 

 require it. 



The practical side of the question may be regarded as having 

 been settled long ago by farmers, since the very great majority of 

 cows which provide the milk-supply of the country are allowed to 

 become pregnant usually within a few weeks (sometimes rather 

 later) of parturition. 



The rise in the non-fatty solids may perhaps be accounted for 

 by the same suggestion as in the case of the rise of nitrogen. 



The sugar content is extremely constant throughout the whole 

 period of lactation. 



The increase in the fat is so slight that in view of the general 

 wide variations in fat content, it hardly calls for discussion. 



The ash content of cows' milk in relation to the period of lacta- 

 tion was exhaustively examined by Trunz in two cows. He found 

 that no definite alterations in the salt content could be detected, 

 although generally there was a slight fall in the first weeks, which 

 rose again towards the end of the tenth month. 



Variations in the composition of milk in relation to the amount 

 of milk given will be considered in dealing with the individual 

 constituents. 



C. Composition of Milk in the Early Days after Parturition (Colos- 

 trum). The value of colostrum to the young animal of the same 

 species would appear to be very great. Naturally-fed infants and 

 calves each receive the colostrum of their own species, but the 

 colostrum of the cow is not a marketable product, and, as far as 

 I am aware, there is no record of its having been supplied to infants 

 as a food. 



The composition of colostrum is, however, a matter of con- 

 siderable intrinsic interest, and a brief digression upon this subject 



1 Cp. Engel and Murschhauser, p. 42. 



