Among animals, in ordinary times, the sexual appetite is completely in abeyance; it is only awakened when the conditions for conception are particularly favorable. The sexual excitement in the female appears at the time of the rut; at a time when ovulation, a muco-sanguineotts dis charge, and the sexual instinct, occur coincidently. But this time is peculiarly fitted for the reception of the semen; the swelling of the exter nal genitals and the vagina occasion a great expansiveness of these por tions, and in consequence permit of an easier and deeper penetration of the male organ. The secretion, the analogue of the menstrual blood, shows an alkaline reaction, and is, from a veterinarian point of view, peculiarly servic,eable for the preservation of the vitality of the semen. The cervical cavity, in distinction to the time outside of the rutting period, is markedly dilated by the swelling, and therefore favors the passage of the semen in a marked degree, a condition to which much value is at tached by vet,erinary surgeogs. The immediately recurring rut, which in animals (in the horse, as e,arly as the eighth day) occurs immediately after delivery, also favors the recurrence of gravidity. It is only during or upon the accession of this rutting period, whether it is immediately be fore or after, that the female allows of cohabitation. Ovulation and co habitation occur therefore at the same time.
In the human species, this is different. The sexual appetite is perma nent in both sexes, and is not dependent upon occurrences in the genera tive organs. Cohabitation occurs irregularly, is performed at all times, and seldom occurs at the time of ovulation; in fact at the time at which the extrusion of ova takes place, which we must look upon as correspond ing to the rut in animals, the menstrual period, cohabitation is most avoided. It is probable that the unfruitfulness of numerous marriages depends upon the fact that cohabitations do not correspond to ovulation. Perhaps certain other processes in the human male genitals aro to be re garded differently than our knowledge of those same processes in animals would seem t,o indicate. That an ovum is extruded at each menstrual period in the human species has certainly not been anatomically proven as yet; it is probable, however, that the menstrual congestion may occa sion the enlargement of a follicle, and bring about its advancement to the surface of the ovary, and that its dehiscence may follow at the next period, so that the possibility of conception may indeed extend to greater intervals than exactly four weeks. Single cases from Leopold's series of investiga tions at least favor this supposition. If, however, ovulation may occur at irregular intervals and is entirely independent of menstruation, as many at the present time are inclined to believe, then the result of coitus, par ticularly when this is seldom performed, is entirely fortuitous. Just as
little do we know positively of the duration of the migration of the ovuni through the tube and of the vitality of the semen. But if, however, the human ovum passes more rapidly through the tube, and human semen retains its vitality for a less time in the female genitals, than we know to be the case in certain animals, then the chances of the meeting of the two eletnents are still less.
With these conditions, temporarily so unfavorable to prompt concep tion, many local factors probably concur. In most of the more highly organized animals, the connection between the tube and ovary is a more intimate one than in the human species. In many animals the ovary is situated in a more or less closed capsule, which catches up the ovum ex truded by the rupture of the follicle and transmits it into the tube, so that the latter at first sight proclaims itself the excretory duct of the ovary. In others the ovary lies in a fold or sulcus of the peritoneum, which only incompletely takes the place of the capsule mentioned above. The apparatus for the transportation of the ovum in the human species is only shared by a very few mammalia. In the place of the fold or sulcus, there appears a pocket, formed by the peritoneal fold which the ovary carries, and the anterior surface of the ovary and the ligamentum latum. According to the testimony of Henle, this pocket is the most incomplete of all in the human species. By means of this extremely defective ar rangement, the ovum is compelled to traverse the open abdominal cavity, and is here exposed by many contingencies to the danger of a deviation from the path to its destination. For instance, every unfavorable move ment of the intestines may remove the ovum from the abdominal opening of the tube, but if it is once removed from this opening, there is little probability, on account of the lack of automatic movement, of its ever again reaching it. Impregnated or unimpregnated, it is usually destroyed in the abdominal cavity. But admitting that theory, which declares that the ovum can be taken up by the above-described erection of the fimbriated end of the tube, to be correct, the pavilion of the tube can still cover only a portion of the surface of the ovary; if now a follicle rup tures at any other portion, the ovulation will be without result. Many other reasons could be cited, which would help to explain the delayed and infrequent occurrence of conception; for instance, in the newly married, the incomplete sexual union occasioned by various mechanical causes; in pluripam, the unfavorable conditions of the external genitals for the proper retention of the semen, and the direction of the vagina, etc., causes which can hardly be taken into account when speaking of animals.