DIFFICULT PARTURITION FROM NARROW PELVIS.--A disproportion between the fetus got by a large stallion and the pelvis of a small dam is a serious obstacle to parturition, sometimes seen in the mare. This is not the rule, however, as the foal up to birth usually accommodates itself to the size of the dam, as illustrated in the successful crossing of Percheron stallions on mustang mares. If the disproportion is too great the only resort is embryotomy.
FRACTURED HIP BONES.--More commonly the obstruction comes from distortion and narrowing of the pelvis as the result of fractures. (Plate XIII, fig. 2.) Fractures at any point of the lateral wall or floor of the pelvis are repaired with the formation of an extensive bony deposit bulging into the passage of the pelvis. The displacement of the ends of the broken bone is another cause of constriction, and between the two conditions the passage of the fetus may be rendered impossible without embryotomy. Fracture of the sacrum (the continuation of the backbone forming the croup) leads to the depression of the posterior part of that bone in the roof of the pelvis and the narrowing of the passage from above downward by a bony ridge presenting its sharp edge forward.
In all cases in which there has been injury to the bones of the pelvis the obvious precaution is to withhold the mare from breeding and to use her for work only.
If a mare with a pelvis thus narrowed has got in foal inadvertently, abortion may be induced in the early months of gestation by slowly introducing the oiled finger through the neck of the womb and following this by the other fingers until the whole hand has been introduced. Then the water bags may be broken, and with the escape of the liquid the womb will contract on the solid fetus and labor pains will ensue. The fetus being small will pass easily.
TUMORS IN THE VAGINA AND PELVIS.--Tumors of various kinds may form in the vagina or elsewhere within the pelvis, and when large enough will obstruct or prevent the passage of the fetus. Gray mares, which are so subject to black pigment tumors (melanosis) on the tail, anus, and vulva, are the most liable to suffer from this. Still more rarely the wall of the vagina becomes relaxed, and being pressed by a mass of intestines will protrude through the lips of the vulva as a hernial sac, containing a part of the bowels. If a tumor is small it may only retard and not absolutely prevent parturition. A hernial protrusion of the wall of the vagina may be pressed back and emptied, so that the body of the fetus engaging in the passage may find no further obstacle. When a tumor is too large to allow delivery the only resort is to remove it, but before proceeding it must be clearly made out that the obstruction is a mass of diseased tissue, and not a sac containing intestines. If the tumor hangs by a neck it can usually be most safely removed by the écraseur, the chain being passed around the pedicel and gradually tightened until that is torn through.