_Liquor amnii._ The amnion contains a fluid known by the name of liquor amnii. In the earlier months of pregnancy it is nearly, if not quite transparent; as pregnancy advances it becomes turbid, containing more or less of what appears to resemble mucus: it has a distinctly saline taste; its specific gravity is rather more than that of water. Its relative and absolute quantity vary considerably at different periods of pregnancy: thus the relative weight of liquor amnii to that of the foetus is very considerable at the beginning of pregnancy, at the middle they are nearly equal, but towards the end, the weight of fluid to that of the child, diminishes considerably, so that during the last weeks of pregnancy it scarcely equals a pound, and seldom more than eight ounces, whereas the medium weight of the child is usually between six and seven pounds: the quantity, however, varies considerably, sometimes amounting to several quarts. In the early months the absolute quantity increases, so that between the third and fourth months it sometimes equals as much as thirty-six ounces. Chemically it consists chiefly of water, a small quantity of albumen and gelatine, a peculiar acid called amniotic, with a little muriate of soda and ammonia, and a trace of phosphate of lime. The source of the liquor amnii is still unknown. Dr. Burns asserts that "it is secreted from the inner surface of the membrane by pellucid vessels," but as he confesses that "these have never been injected or traced to their source (_Principles of Midwifery_, by J. Burns, M. D. p. 222.,) little weight can be attached to such a view." Meckel considers (_Handbuch der Menschlichen Anatomie_, vol. iv. p. 707,) that the greater part of it, especially in the early months, is a secretion from the maternal vessels, but that afterwards, as pregnancy advances, it becomes mingled with the excretions of the foetus. It appears to be a means of nourishment to the foetus during the first part of pregnancy, from the fact that it contains more nutritious matter in the early than in the latter months, since at that time a considerable coagulation is produced by alcohol, &c. The disappearance of this coagulable matter of the liquor amnii, towards the end of pregnancy, may be attributed to its having been absorbed at an earlier period, and to the process of nutrition being now carried on by other means. Besides being a source of nourishment to the foetus, it serves many useful purposes; it secures the foetus against external pressure or violence, and supports the regular distension of the uterus; on the other hand it diminishes and equalises the pressure of the foetus upon the uterus; during labour by distending the membranes into an elastic cone, it materially assists to dilate the os uteri; it also serves to lubricate and moisten the external passages.