_Semen._—The semen is composed of the secretions of the testicles, seminal vesicles, prostatic gland, Cowper’s glands and the urethral glands. The semen is a thick, colorless, gelatinous, opalescent, non-transparent, viscid fluid, resembling boiled starch. It possesses a specific odor sui generis, due to the presence of phosphate of spermin. Its specific gravity is higher than that of water, its reaction is alkaline. It is soluble in water and acids, and is coagulated in alcohol. If it is let stand in a test tube, two layers will be formed, a lower one opaque, consisting of spermatozoa and other cellular elements, and an upper one turbid and translucent, with only a few cells and detritus. The two layers are of about equal thickness. The semen consists of about ninety per cent. water and ten per cent. solids. Of the solids sixty per cent. are organic substances, thirty per cent. earthly phosphates, and ten per cent. sodium chloride. When a drop of fresh semen is observed under the microscope, it is found full of motion, as if an anthill had been stirred up. This motion lasts for about twelve hours. It is caused by the living spermatozoa. The number of spermatozoa in an ordinary emission, of about ten grammes, is about two hundred to three hundred millions. Besides the moving spermatozoa, there are found a certain number of lecithin globules. Their size is about half the size of a red blood corpuscle. When semen is let stand for a certain time, Boettcher’s crystals, consisting of phosphate of spermin, are formed from the base which gives to the semen its peculiar smell and which is derived from the prostatic secretions. The semen also shows the presence of different kinds of epithelia, the pavement epithelia from the fossa navicularis urethra, the round cells from the prostate, and the columnar epithelia from Cowper’s glands. _Erection._—The male sexual activity consists in the essential features, intromission and ejaculation. For intromission erection is an absolute necessity. The lumen of the vagina is only virtual, there is no real lumen. It follows that only an unflexible body could penetrate into this organ, where the semen has to be deposited for the production of the new being. For this reason, it is prerequisite to copulation that the penis, which is normally in a state of flaccidity, should obtain the required rigidity. This rigidity is gained in the following way. The tonus, which is present in all the blood-vessels of the body, is the cause that the arteries of the corpora cavernosa penis have only a virtual and not a real lumen. For between the layers of the circular muscular fibres of these vessels is found a layer of longitudinal fibres which narrow the lumen and almost entirely compress it.