This disease is discussed in the chapter on "Infectious Diseases."
As the period of parturition approaches, the swelling of the udder bespeaks the coming event, the engorgement in exceptional cases extending forward on the lower surface of the abdomen and even into the hind limbs. For about a week a serous fluid oozes from the teat and concretes as a yellow, waxlike mass around its orifice. About 24 hours before the birth this gives place to a whitish, milky liquid, which falls upon and mats the hairs on the inner sides of the legs. Another symptom is enlargement of the vulva, with redness of its lining membrane, and the escape of glairy mucus. The belly droops, the flanks fall in, and the loins may even become depressed. Finally the mare becomes uneasy, stops feeding, looks anxious, whisks her tail, and may lie down and rise again. In many mares this is not repeated, but they remain down; violent contractions of the abdominal muscles ensue; after two or three pains the water bags appear and burst, followed by the fore feet of the foal, with the nose between the knees, and by a few more throes the fetus is expelled. In other cases the act is accomplished standing. The whole act may not occupy more than 5 or 10 minutes. This, together with the disposition of the mare to avoid observation, renders the act one that is rarely seen by the attendants.
The navel string, which connects the foal to the membranes, is ruptured when the fetus falls to the ground, or when the mare rises, if she has been down, and the membranes are expelled a few minutes later.
When there is a single foal, the common and desirable presentation is with the fore feet first, the nose between the knees, and with the front of the hoofs and knees and the forehead directed upward toward the anus, tail, and croup. (Plate XII, fig. 1.) In this way the natural curvature of the body of the fetus corresponds to the curve of the womb and genital passages, and particularly of the bony pelvis, and the foal passes with much greater ease than if placed with its back downward toward the udder. When there is a twin birth the second foal usually comes with its hind feet first, and the backs of the legs, the points of the hocks, and the tail and croup are turned upward toward the anus and tail of the mare. (Plate XII, fig. 2.) In this way, even with a posterior presentation, the curvature of the body of the foal still corresponds to that of the passages, and its expulsion may be quite as easy as in anterior presentation. Any presentation aside from these two may be said to be abnormal and will be considered under "Difficult parturition."