This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
The Period Of Gestation In The Human Species is about two hundred and seventy days, but is undoubtedly liable to a certain amount of variation. Toward the end of that time, a great relaxation of parts takes place, even the ligaments which hind the bones of the pelvis being considerably slackened; the cervix uteri begins to dilate, and the muscular walls of the uterus are seized with recurrent spasmodic contractions. Usually the uterine contractions burst the membranes which retain the liquor amnii. and afterwards the child is expelled, its head first, and is shortly followed by the placenta and ruptured membranes. In healthy parturition, the contraction of the uterus is sufficient to prevent much bleeding ensuing from the tearing across of the blood-vessels which united the placenta with its substance. Forthwith, the uterus begins to undergo a rapid process of involution. Its muscular walls, which had undergone great enlargement, both by increased size of the individual fibres, and continual growth of others from connective-tissue corpuscles, become daily diminished by degeneration and disappearance of the exaggerated fibres; and the organ returns to its original proportions.
218. Within a day or two after the birth of the child, the breasts of the mother, which have previously been enlarging, come into functional activity, and there is a copious secretion of milk.
The breasts, or mammas, have their proper secreting structure, at other times, so closely connected with the areolar tissue in which it is imbedded that it is difficult to trace; but during lactation, it is much more distinct. From about fifteen to twenty separate ducts open at the extremity of the nipple; each of these is dilated into a small reservoir between one and two inches from its extremity, and, traced further back, is found to branch repeatedly, till the radicles are reached, connected with the ultimate lobules, which consist of aggregations of rounded secreting saccules.
Milk, examined microscopically, is a clear fluid, with oil globules of many different sizes floating in it. In the process of ohurning, the oil globules of milk are thrown together in a solid mass, and constitute butter; while in milk which is allowed to remain at rest, so that the larger globules rise to the top without running together, the stratum in which these Accumulate is the cream. The solid constituents of milk are casein, butter, sugar of milk, and a small quantity of salts. As compared with the milk of the cow, that of the human species has a smaller proportion of solid matter, and the solid matter contains a third less of casein, twice as much sugar, and not half the quantity of salts; and on that account, in feeding infants with cows' milk, it is customary to mix it with sugar and water.
The first milk which is secreted after parturition exercises a purgative influence on the infant, and is termed colostrum; it is characterised by the presence of corpuscles consisting of heaps of granules gathered together into balls. After prolonged lactation the milk secreted becomes poor in quality, and the continuance of suckling becomes hurtful, both to the mother and to the child; suckling for an inordinate length of time is, therefore, to be avoided. The best food, however, for the new born infant is the mother's milk; and no more disgraceful custom can well be imagined than a healthy mother neglecting the duty of suckling, unless it be that of medical practitioners encouraging such an impropriety.