=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. =Erection. The Corpus Cavernosum.=--The most curious part of this apparatus is the mechanism of _erection_, or the power possessed by the penis of swelling under the influence of certain nervous irritations, increasing in length and diameter as well as becoming rigid. This phenomenon is produced by three organs called the _cavernous bodies_ which form the principal bulk of the penis. One of them, situated in the middle and underneath and formed by two bodies united into one, surrounds the urethra and terminates in front in a dilatation which constitutes the glans already mentioned. The two others are situated symmetrically on the dorsal part of the penis. All three consist of caverns or diverticula formed by blood-vessels, which are empty when the penis is flaccid. By a complex nervous mechanism based on vascular paralysis due to nervous phenomena called inhibition and dynamogeny, the nervous irritations cause an accumulation of blood in the spaces of the cavernous bodies which become so gorged with blood as to form stiff and hard rods. The size of the penis is thereby increased considerably and its stiffness allows it to penetrate the vagina of the female. At the same time and by the same mechanism the verumontanum swells so as to close the ureter from the bladder, while the seminal ducts open toward the urethral orifice. In this way the copulatory organ is ready for its function.