Secreted by mother 



902 THE EXTERNAL SECRETIONS 



3-4 weeks 572 grams 



5-8 weeks 736 grams 



9-12 weeks 797 grams 



13-16 weeks 836 grams 



17-20 weeks 867 grams 



21-24 weeks 944 grams 



25-28 weeks 964 grams 



29-32 weeks 916 grams 



33-36 weeks 909 grams 



37 weeks 885 grams 



The fats of milk are similar to those contained in adipose tissue. Their propor- 

 tion may be given as follows: olein, ^^, palmitin, 3^; stearin, 3^ ; butyrin, caproin, 

 and caprylin, 3^4. In milk-plasma are found various proteins, a carbohydrate, 

 lactose, inorganic salts and a small amount of lecithin and nitrogenous extractives. 

 The principal protein is called caseinogen. It belongs to the group of the phos- 

 phoproteins and may be precipitated by acids, such as acetic acid, or by saturation 

 with magnesium sulphate, or half-saturation with ammonium sulphate. Rennin 

 causesit to coagulate with the formation of casein. This process is employed in the 

 preparation of cheese, the curd consisting of casein and entangled fat-globules. 

 If this coagulated mass is allowed to stand, milk-serum separates from it. The 

 latter contains two other proteins, namely, lactalbumin and lactoglobulin. The 

 carbohydrate is milk-sugar or lactose, a disaccharide of the composition: C12H22O11. 

 When hydrolized it takes up water and galactose : 



Ci2H22Hii + H2O = C6H12O6 + C6H12O6 



It may be also be found in the urine of woman during the early days of lactation, 

 when there is a reabsorption of the lactose owing to an insufficient withdrawal 

 of the milk. For the same reason, it may enter the urine during and after the period 

 of weaning. It then gives the ordinary tests for reducing sugar. The salts of 

 milk consist of calcium phosphate, a small quantity of magnesium phosphate and 

 the chlorids of sodium and potassium. Especially marked is the richness of 

 milk in calcium, phosphorus, and magnesium, as compared with the blood-serum. 

 This is of greatest importance for the growth of the bones, while the growth of the 

 tissues necessitates potassium rather than sodium. At all events, it is a most 

 striking fact that these cells are capable of selecting from the fluids of the body 

 only those salts which are of greatest use to the developing young. This selective 

 action they also display in the formation of caseinogen and lactose, both of which 

 do not exist as such in the blood or lymph. It is by means of this concentration of 

 materials of the proper kind that the rabbit is enabled to double its weight in six 

 days, the dog in 96 days, and the infant in 108 days. 



The practical importance of these brief chemical data becomes 

 apparent immediately if a substitute must be sought for human milk.^ 

 Although other types of milk are more like human milk, we are then 

 accustomed to use cow's milk, because it is most easily procured. Its 

 composition, however, is very unlike that of human milk, as the 

 following tabulation will show: 



Human Cow's 



Water 87.16 87.10 



Fat 4.28 4.20 



Casein 1 .04 3.25 



Sugar 7.40 5.00 



Ash 0.10 0.52 



^ Voltz, in Oppenheimer's Handb. der Biochemie, 1910, iii, 382. 



