It is possible that some irregularities in the position of the ovum of mammalia during ges tation may receive an explanation from mecha nical disturbances similar to those we have now mentioned in birds ; for supposing that in a viviparous animal the ovum does not gain the uterus or usual place of its abode during gesta tion, development of the foetus still takes place. In those instances in which a foetus is formed in the region of the ovary, or in what are termed ovarian conceptions, for example, it is not pro bable that the ovum is ever developed in the ovary itself without the bursting of the Graafian vesicle : it may be fixed close to the ovary, but it is always independent of that body. After the Graafian vesicle has burst, the ovum may be supposed either not to have been received in the Fallopian tube, or, after having entered that passage, to have been expelled from it by an inverted action of its muscular fibres or other causes. Fecundated by the contact of some of the seminal fluid which has reached so far into the Fallopian tube, the ovum remains in the neighbourhood of the ovary, has a cyst formed round it, and becomes organically united to the ovary or parts in its vicinity by structures similar to those which unite the ovum to the inner surface of the gravid uterus ; for the bloodvessels of the mother which run into the cyst enlarge and form a placenta by their union and intermixture with those of the foetus, and thus for a considerable time (amounting sometimes to four or five months) this ovarian or extra-uterine gestation is carried on.
In other instances of misplaced gestation, the ovum seems to have been arrested in its course when more or less advanced in the Fallopian tube ; but here also the parts are susceptible of all those remarkable changes and growth which favour the development of the foetus in the ovum. We mention these instances of extra uterine gestation with the view of directing the reader's attention to an inference which may be drawn from them, viz. that all those changes of growth upon which the development of the ovum in viviparous animals depends may he regarded rather as belonging to the ovum itself than as resident in the uterus alone. It is worthy of remark, however, that in ovarian and tubular conceptions the decidua is formed within the uterus, nearly in the same manner as if the ovum had descended in the natural way into its cavity; from which we may infer that the production of the decidua is to be re garded as one of a series of changes induced by conception in the internal genital organs, and occurring independently of one another, rather than as the effect of any stimulation from the ovum, as some have supposed. Such a decidua in fact may be compared to the sub ventaneous ovum of the bird.
Very little is as yet known as to the physical circumstances (independent of malformation of the organs) which may give rise to misplaced gestatinn ; and this is not a subject which we can hope to have illustrated by observation or experiment. One or two cases are on record,
however, from which it might appear possible that a violent disturbance of the body soon after sexual union may be a cause of misplacement of the ovum. Burdach mentions instances of this kind : one of a cow gored by the horns of another soon after copulation, and two instances of the human female in which sudden fright in the same circumstances was followed by ovarian conception.' In endeavouring to apply such mechanical explanations, we ought not to forget that in by far the greater number of cases sudden motion does not appear to disturb the natural perform ance of all those actions by which the ovum is securely lodged in the uterus in the natural way.
Circumstances influencing the liability to eonception.—The circumstances which influence the liability of the female to conception are so various and so little determined that our re marks on this subject must be very short.
The healthy condition of the female is of course an important circumstance in reference to conception, but we do not know in how far a robust constitution or high state of health is favourable or the reverse to the occurrence of conception. Some women, it would appear, (perhaps those of a spare habit of body and languid powers of constitution) are most liable to fall with child when in their strongest and best state of health, while weakness in others seems to induce conception. Among animals it is known that high feeding sometimes pre vents pregnancy, and the same is the effect of the opposite extreme of starvation.
The regularity of the menstrual discharge is one of the most important circumstances which favours the liability of women to conception ; perhaps more from its being an indication of the general healthy state of the generative or gans than from any influence exerted by the menstrual change itself. Many circumstances, however, seem to render it probable that women are more liable to conception within a few days after the cessation of the menstrual flow than at any other period of the interval, and accordingly there are many accoucheurs who regulate their calculations of the time of birth from this cir cumstance, dating the commencement of utero gestation from a period within a week after the cessation of the last menstrual discharge. We do not know with certainty upon what circum stances this influence of the menstrual function depends ; but it seems reasonable to suppose that it is connected with that state of excitement and sanguincous congestion in the ovaries and rest of the generative organs which usually at tend on menstruation. There seems to be very little reason to believe, as some do, that there is a greater than ordinary liability to concep tion immediately before the commencement of menstruation.