It would seem that sterility in the female must depend upon one of several causes. The ovaries may be incapable of forming eggs, or the eggs when formed are defective and incapable of fertilization. The ovaries, on the other hand, may be functionally perfect, but owing to some obstructive condition of the Fallopian tubes or oviducts they fail to reach the uterus. Again, a perfectly healthy ovum may be impregnated and safely conveyed to the womb, but unless that organ is in a normal condition it may die, conception would not take place, and the mare would as a consequence fail to breed.
The writer has known several instances where the entire structure of the egg-forming glands have been destroyed by the growth and expansion within them of cysts or bladder-like formations (fig. 527), and other cases where the glands have been rendered functionally useless by the development within them and round them of malignant tumours. The Fallopian tubes may be rendered impervious by pressure from without, or by thickening of the membrane lining them, or by morbid growths within them or upon them. Mares which have passed through a period of difficult foaling not unfrequently become sterile owing to the Fallopian tubes getting blocked up by inflammatory products, or so far thickened as to obliterate the passage and prevent the ovum from reaching the uterus.
However perfect the ovaries or oviducts may be, impregnation cannot take place unless the semen of the male gains access to the uterus, for which purpose it is necessary that the entrance thereto should be open to receive it.
Obstruction at this point is not unfrequently the cause of sterility in mares, either as the result of a twist of the neck of the uterus, or a thickening of its walls, or disease of the mucous membrane, any one of which may obstruct the passage and prevent the entrance of the sperm element into the womb.
From these considerations it will be seen that the possibility of restoring fruitfulness in the sterile mare will depend upon the nature of the cause to which the sterility is due. Of these some are amenable to treatment, but others are altogether incapable of being removed. While it would be impossible to restore the function of an ovary or egg-gland whose structure had been broken up and absorbed by the growth of cysts or some other formation within it, it might not be difficult to remove or overcome an obstruction in the neck or mouth of the uterus, or, in some cases, to restore its lining membrane to a normal condition.
In order that an impregnated ovum may proceed to develop into a foetus, the womb with which it must establish a connection will require to be in a healthy condition. Many mares fail to breed, not from any structural defect of the reproductive organs, but from a functional derangement of the mucous membrane of the uterus or vagina, whose vitiated secretion imperils, if it does not immediately destroy, the life of the spermatozoa, or should they escape and impregnation take place, the fertilized ovum sooner or later succumbs to its unhealthy environment. Many of those cases where mares return to the horse and receive service again and again without proving fruitful, result from some one or other abnormal state of the uterus unfitting it to nurture the impregnated germ.
Although little can be done to rectify those graver structural defects of the ovaries and the uterus which add to the prevalence of sterility, much may be done to prevent that greater waste resulting from obstructive conditions affecting the mouth and neck of the womb, which prevent the semen from entering it.
It has repeatedly been affirmed and implied that in the act of coition the spermatic fluid of the horse is deposited in the vagina of the mare, and that the spermatozoa subsequently enter the uterus by virtue of their own powers of movement or are sucked into the latter organ during its relaxation, when copulation is completed. While allowing the operation of both these forces in the act of insemination, it is impossible to disregard the mutual adaptation of the male and female organs to the purpose of conveying the semen directly into the mouth of the womb. The projection of the urethral canal beyond the glans penis in a state of erection would seem to indicate that this arrangement was designed to ensure the delivery of the male element into the mouth of the uterus.
That this should take place is not absolutely necessary to fertilization. It has been proved by experiment that the injection of semen into the vagina alone may be sufficient to induce pregnancy. Because this is so, it has been argued that the spermatic fluid in the act of copulation in the horse is not discharged into the uterus. It seems to the writer that such a conclusion is not warranted by the facts.
That the introduction of semen into the vagina is followed by pregnancy does not exclude the possibility of its being deposited directly into the mouth of the womb during the act of coition, but would rather appear to afford a supplementary provision for impregnation in the event of this not being effected.
Whether insemination is brought about by one method or the other, or both, a clear entrance to the uterus is an indispensable condition to impregnation by natural means.