may reach the ovum awaiting impregnation. It is easily understood that all of the spermatozoa which are deposited in the vagina do not reach their goal, the ovum. Part of the semen follows the virile member upon its withdrawal. But even later a .further portion of the fluid is lost, and so much the more so, as the closure of the genitals externally is too often extremely imperfect. But even with complete retention of the semen, only a small number of spermatozoa pass through the external os. For the latter, without will and without instinct, separate irregularly in all directions, so that only a small number conic in direct contact with the external os. Another portion perhaps occasionally reaches the external os in a circuitous way, after it has encountered some obstacle on the walls of the vaginal vault; but it is certain that the greatest portion, exhausted by the prolonged fruitless movement, is destroyed in the vagina.
The narrowness of the cervical cavity should be favorable rather than unfavorable to the onward movement of the spermatozoa,,although here again the folds certainly prevent a part from moving further. A great
number of spermatozoa are certainly lost in the roomy uterine cavity, so that they never reach the extremely narrow tubal openings; and of those, which do penetrate into these tubes, half never attain their object, since usually only one ovum is to be found in a tube. In this way the number of the spermatozoa, in their long migrations and from the relatively short duration of life, must decrease in a rapidly increasing progression. Bat since, on the one hand, the number of spermatozoa is an enormous one, and on the other hand only a single spermatozoon is necessary for the im pregnation of the ovum, and as it is perhaps probable that impregnation can only occur by the conjunction of a single spermatozoon with a single ovum, the ejaculated semen is, under proper conditions, efficient for con ception. It is different, however, in those cases in which the distance is increased, the obstacles, greater and more numerous, the ejaculated semen small in quantity and containing but few spermatozoa; under these cir cumstances repeated cohabitations may remain fruitless.