Milk when freshly drawn from the breast is a warm, sweet, sterile, alkaline fluid with a specific gravity of about 1030. The chief solid ingredients,1 which amount to 13 per cent, are proteins (1-2 per cent), fat (3-4 per cent), sugar (5-7 per cent), and salts (about 0.2 per cent). While the fluid part (87 per cent). of milk is most essential to the infant, it may be disregarded at present as not possessing any properties other than those of water. The solids on the other hand are the chief tissue builders, and the source of heat and energy. For the proper growth and development of the infant a certain proportion of each of these solid constituents must be present. For the growth and repair of tissues, proteins and mineral salts are essential; for the production and storage of heat, fat is necessary; and the source of energy lies in the sugar. The proteins are in the form of caseinogen and lactalbumin, the fat is represented by cream, and the carbohydrate present is milk sugar. Breast milk contains these substances in the proportion which is best suited to an infant's digestive powers, which with the large demands of a growing child are to be kept fully employed but must not be overtaxed. The proteins are much more digestible than those of cows' milk which form dense hard curds, and consequently protein-indigestion is not a common trouble in breast-fed infants. The fat is in the form of a fine emulsion which as a rule is easily digested and absorbed. The sugar is very easily absorbed, and is seldom the cause of any gastric disturbance. There is considerable difference of opinion as to the exact chemical composition of milk, and both the nature of the proteins and the effect produced on them by the acids and ferments of the stomach are not yet thoroughly understood. The quantity of milk secreted by the breast varies according to the requirements of the infant. The secretion is not established usually until the third day after parturition. It differs for a few days from the normal standard and is called colostrum. The colostrum contains some special protein substances, and is also supposed to act as a laxative. The amount of milk is gradually increased to meet the requirements of the child. The following estimates by Pfeiffer as to the daily amount are quoted from Holt.
1 The term "composition of milk" refers to the chemical composition as far as it can be ascertained. There are of course many other ingredients, organic compounds, etc., which have not been determined, but which play an important part in the human economy.
At the end of 1st week ....
During the 2nd week ....
,, 3rd ,, ....
„ 4th „ ....
From the 5th to the 13th week
„ 4th „ 6th month .
6th „ 9th
With this may be compared the amounts required as estimated by Feer (quoted by Hutchison).
At the end of 1st week .... 291 grms.
During the 2nd week .... 549 „
During the 3rd week .... 590 „
During the 4th week .... 652 „
From the 5th to the 13th week . . 687-828 grms.
From the 4th to the 6th month . . 893-980
The rapid increase in the amount secreted during the first few weeks of life is very striking, but corresponds with the rapid growth of the infant at that period.