In the higher mammals the womb becomes more and more developed, opening into a single vagina in the middle line of the abdomen, between the two ovaries, and constituting a highly specialized organ which allows the mother to preserve the young for a long time in her belly. In most mammals the uterus has two elongated diverticula, each of which may contain a successive series of embryos. In man it forms a single cavity and normally contains a single embryo, occasionally two or more. These facts show that the role of the female mammal in reproduction is more important than that of the male. But this is not all. Whether it still lays eggs, or whether it gives birth to young which are more or less developed its sexual role is far from ended. The higher oviparous vertebrates, especially the birds, take care of their progeniture for some time after laying. The young are still fed by the mother, either by milk from the teats, as in mammals, or by nourishment obtained from outside, as in birds, or by both methods combined or succeeding each other, as in cats.
In many animals the male contributes to the raising of the young; a point to which we shall return. Here, we indicate these complicated details simply to show that sexual union only contributes one link in the long chain of reproduction. Let us study its mechanism in man.
=The Copulatory Organ of Man. The Testicles. The Seminal Vesicles.=--Nature is often very sparing even in the highest organizations. It has thus combined in the male the urethra with the copulatory organ, and the sexual germinal glands, or _testicles_, with an accessory gland, the _epididymis_. Hundreds of thousands of spermatozoa are contained in the glandular tubes of these organs, which, when they are mature can always produce new ones by cell division. The spermatozoa accumulate at the extremity of the duct of the gland in a reservoir called the _seminal vesicle_, where they float in the mucus, thus constituting the seminal fluid or _sperm_. This liquid has a special odor. The two seminal vesicles are situated in the abdominal cavity underneath the urinary bladder, each having a duct which meets that of the other side and opens by the side of it in the deep part of the urethra. Here the secretion of several other glands, especially of the _prostate_, is added to the sperm and mixes with it. The point where the two seminal ducts open into the urethra forms a small elevation, the _verumontanum_. From this point the male urethra emerges from the abdominal cavity and is continued along the special prolongation which forms the penis, or virile member of copulation. In the ordinary way the penis only serves for the emission of urine. It hangs flaccid and terminates in a rounded swelling called the glans, at the end of which opens the urethra (Fig. 18). This opening serves also for the emission of the sperm.