REPRODUCTION 



1021 



food proper, on the tissues of the mother, and that in as literal a 

 sense as when it drew its supplies directly from the maternal blood. 



Milk. The milk secreted during the first few days of each lacta- 

 tion, the colostrum, as it is called, indeed may represent in part the 

 fragments of cells lining the alveoli of the mammary glands, which 

 have undergone a fatty change and been bodily broken down. The 

 colostrum corpuscles are leucocytes filled with fat globules taken up 

 from the contents of the alveoli. The chief chemical difference 

 between colostrum and ordinary milk is the greater richness of the 

 former in protein. It has been supposed that it is of special impor- 

 tance for the nutrition of the suckling, perhaps in virtue of the 

 enzymes contained in it, and it is said that young animals bear 

 artificial feeding much better if they have been allowed to suckle the 

 mother for the colostrum. In addition to the fat, which when milk is 

 allowed to stand rises to the top as cream, milk contains a consider- 

 able quantity of caseinogen, to whose coagulation, under the influence 

 of the lactic acid produced from the lactose, or milk-sugar, by certain 

 bacteria, spontaneous curdling is due. Another protein, lact-albumin 

 (Halliburton), a large amount of water, and some inorganic salts, are 

 the most important of its remaining constituents. The molecular 

 concentration (p. 398) of milk, as measured by its freezing-point, is 

 almost exactly the same as that of blood-serum. Its electrical 

 conductivity varies extremely, since it depends on the quantity of 

 fat present, the fat globules, like the blood-corpuscles, being practi- 

 cally non-conductors. 



The inorganic composition of milk is particularly interesting when 

 compared with that of the blood on the one hand and that of the 

 suckling on the other. Thus, 100 grammes of ash from each source 

 gave the following values for the rabbit (Abderhalden) : 



The richness of the milk (and of the suckling) in calcium, phos- 

 phorus, and magnesium, as compared with the serum, is to be 

 especially remarked. This is, of course, essential for the develop- 

 ment of the bones. Whereas sodium predominates greatly over 

 potassium in the serum, the opposite is the case in the milk (and the 

 suckling) . This is connected with the development of the tissue cells, 

 which are richer in potassium than in sodium. The high chlorine 

 content of the serum is in sharp contrast with the relative poverty of 

 the milk in that element, which preponderates in the tissue liquids 

 and is relatively scanty in the cells. 



In addition to substances susceptible of chemical analysis, milk 

 contains enzymes like those present in blood-serum, including 

 oxydases and various hydrolytic ferments (proteolytic, diastatic, 

 and perhaps lipolytic). It is now universally acknowledged that 



