But here the analogy ceases. And from this point onwards the more closely the two functions are compared, the more plainly does it appear that although the cestrus and men struation possess many circumstances in com mon, yet the resemblance endures only for a certain period, more or less brief, while, after this is past, there follows in tnan an inter mediate condition which is not only not com parable with the corresponding intermediate state in animals, but is in many of its essential features the direct converse of this.
For, as already stated, in the mammalia usually by the time that the ovum has reached the uterine extremity of the oviduct, or has entered the uterus, the opportunity for im pregnation is lost, the (estrus is over, and the animal refuses the male : all the conditions immediately necessary to procreation then pass away, and an interval of perfect inaptitude en sues, which is sometimes so remarkable that not only are no ripe ova to be found in the ovaries, but even the male organ ceases to secrete semen. In this series of recurrent periods, marked by irresistible impulse, alter nating with total inappetence for congress, nothing is more evident than that each corre sponds with an internal physical condition, of which it affords a most intelligible explana tion. The appetency occurring and remain ing only as long as congress would be fruitful ; the inappetency returning whenever this would be necessarily infertile.
Now, with regard to the human subject, whatever may be possible during menstruation, yet essentially the intervals of the menstrual acts are the times of fertility in women. And the only question that can arise upon this point is, whether the power of conception ex tends over the whole or over a part only of this interval—a question that has been already considered.
In all that relates, therefore, to the coinci dence of the ovipont with the cestrus of mam mals, the evidence derived from comparative anatomy serves to strengthen the belief in a corresponding correlation between the emis sion of ova and the act of menstruation in the human subject. But in respect of the inter val, the great divergence of the facts here dis played tends to embarrass and perplex rather than to elucidate the question as it relates to man. For it is precisely in this interval that all the circumstances occur vvhich, for want of a consistent explanation, have often thrown a doubt over the whole theory of the direct dependence of menstruation upon ovarian in fluence ; and in elucidating these points, com parative anatomy affords little or no help.
In taking a retrospect of these several facts relating to menstruation and its connection with a corresponding ovipont, an essential distinction should be made between the influ ence of the ovaries in determining the power of the uterus to perform the menstrual act, and any influence which they may have over the periodicity of that function. In all that relates to the former faculty, the power of the ovaries may be regarded as indisputably esta blished. In much that is connected with the latter, there is obviously room for more in formation than we at present possess.
If each separate act of menstruation is de termined by certain modifications periodically occurring in the ovary, it is probable that the essential part of the process is the maturation of an ovum within the follicle, while the process of its emission may be an accidental feature, not always occurring, sometimes hap pening spontaneously, and sometiines caused in the way already suggested, but having nothing necessarily to do with the menstrual act, although the time of its occurrence may materially affect the period of a resulting im pregnation.
The purpose of theflux remains to be con sidered. If the quantity of fluid escaping at each recurrence of menstruation be estimated at three, or possibly five, ounces, and the pro cess is repeated, without interruption from pregnancy, lactation, or disease, once in every lunar month, or thirteen times annually for thirty years, then an aggregate quantity of seventy-two pounds or nine gallons on the for mer supposition, or of a hundred and twenty two pounds or fifteen gallons upon the latter estimate, will have passed from the system in the course of menstrual life, and, so far as this is composed of blood, will have been ap parently entirely wasted.
It is difficult to arrive at a perfectly, satis factory conclusion regarding the purpose of this large loss. For the external escape of blood must be regarded as, to a certain ex tent, an accidental feature in the process of menstruation. That it is not essential to fer tility, is proved by the fact that women some times, though very rarely, breed who do not menstruate ; that the temporary suspension or the menstrual flow during lactation is no cer tain preventive of conception ; and that, oc casionally, young girls become pregnant before the menstrual age has arrived.