Littré.

As previously mentioned, it is in the testicles that the secretion necessary for the reproductive act is prepared. This secretion is evacuated during sexual intercourse, and also during masturbation and involuntary seminal emissions. The testicular secretion is a tenacious fluid. When examined microscopically, it is seen to contain countless spermatozoa, structures about 50 [Greek: m] (1/500 inch) in length, with a thick head and a long filiform tail. They represent the male reproductive cells, which during coitus are introduced into the interior of the female reproductive organs; a single spermatozoon unites with the ovum of the female to form the fertilised ovum. The spermatozoa are formed in the walls of the convoluted seminiferous tubules. The cells lining these tubules are of several different kinds (although in childhood they are not differentiated as they are after the puberal development has taken place). One variety of these cells, the spermatogonia, undergo an increase of size at puberty, and from these spermatogonia, after passing through several intermediate transitional stages, the spermatozoa are formed.

It was formerly believed that the sole function of the testicles was the production of the spermatozoa; recently, however, the opinion has gained ground that these organs have in addition another specific function, that of internal secretion. Whilst the spermatogonia become transformed into spermatozoa, other cellular structures of the testicle, more especially the interstitial cells, produce, it is assumed, the internal secretion of the gland. The constituents of this internal secretion, having been poured into the general circulation, are supposed to give rise to the specific masculine sexual development, and, in particular, to lead to the appearance of the secondary sexual characters. This matter will subsequently be discussed in detail, and here I shall merely add that perhaps none of the proper constituents of the internal secretion find their way into the external secretion of the testicle.

This external secretion of the testicles does, however, receive the admixture of a number of other secretions, to constitute the semen as actually discharged, viz., the secretion of the prostate gland, that of the seminal vesicles, Cowper's glands, and the glands of the vasa deferentia, and perhaps also that of the glands of Littré. The term semen is, indeed, often applied to the secretion of the testicles alone; but to avoid misunderstanding, Fürbringer8 recommends that only the mixed secretion, as actually discharged, should be spoken of as the semen, and that this term should never be employed to denote the testicular secretion alone.

In what has gone before, I have not only described the structure of the male sexual organs, but have alluded also in passing to their functions. These latter must, however, be described more fully. Let us begin with erection, which, as we saw, is due to distension of the penis with blood. How is this distension brought about? It results from stimulation of the erection centre. Until recently, it was supposed that this centre was situated in the lumbar enlargement of the spinal cord; but now, owing to the researches of L. R. Müller, it is believed to form part of the sympathetic plexuses of the pelvis. Stimulation of the centre leads to distension of the penis with blood, and thus to erection of that organ. The stimulation of the centre can be effected in either of two ways.

In the first place, by psychical processes. Thus, in a man, the sight of a woman exercises such a stimulus, the stimulation proceeding from the brain along the spinal cord to reach the centre. The psychical stimulus may also consist of reminiscences. In this way the memory of an attractive woman may be just as effective in causing erection as if she were actually visible at the moment; reading erotic literature may have the same result. When the sexual impulse is perverted, the ideas causing erection will naturally be themselves of a perverse character. Thus, in the homosexual male, erection occurs at the sight or remembrance of a man; in the fetichist, the idea of the fetich is operative—in the case of the body-linen fetichist, for instance, the idea of articles of underclothing.

In the second place, the activity of the erection centre can be aroused by physical stimuli. To this category belong masturbatory manipulations, stimulation of the glans penis and other parts of the genital organs. But other erogenic areas exist, the stimulation of which produces the same results. Among these areas, the buttocks must be particularly mentioned. But individual peculiarities play a great part in this connexion. Thus, in many persons, a slight stimulation of the nape of the neck, of the scalp, &c., has an erogenic effect. In all cases alike, the stimulus is conducted along the sensory nerves to the erection centre, and it is the stimulation of this centre which by reflex action leads to distension of the penis with blood and its consequent erection. The physical stimulus leading to erection may also result from some pathological process, such as inflammation of the penis or of the urethra. Finally, certain internal physiological processes may be the starting-point of the afferent physical stimuli leading to erection; for example, distension of the bladder, and also of the seminal vesicles, and of the seminiferous tubules of the testicle. In addition, it is probable that many of the processes of growth occurring in the reproductive glands act in a similar way. These internal stimuli all pass to the erection centre along the afferent (sensory) nerves, and induce erection by reflex action; and it is important to bear in mind that this effect may result without any direct affection of consciousness by the originating afferent impulses.

Although either kind of stimuli, psychical or physical, acting alone, may give rise to erection, experience shows that in most instances the two varieties co-operate in the production of this effect. Thus, in the sexually mature man, the accumulation of semen in the seminal vesicles gives rise, not only to excitement of the erection centre, but also to voluptuous ideas, and these latter, in their turn, further stimulate the erection centre.

Normally, during coitus, erection is followed by ejaculation. A special nerve centre for ejaculation is also supposed to exist; and the ejaculation centre, like the erection centre, was formerly believed to be situated in the lumbar enlargement of the spinal cord, but recent investigations have shown that it also most probably forms part of the sympathetic plexuses of the pelvis. This centre also may be stimulated either by psychical or by physical stimuli. In normal conditions, however, much more powerful stimuli are needed to cause ejaculation than those which are competent to give rise to erection. For this reason, erections often occur without leading to ejaculation, whereas in normal conditions ejaculation hardly ever occurs without erection. In fact, ejaculation in the absence of erection is almost peculiar to pathological states, and may occur, for instance, in many forms of impotence, in which the ejaculation centre still remains susceptible to stimulation, whilst the erection centre is exhausted. Whereas stimulation of the erection centre exercises its reflex influence through the vasomotor nerves, thus leading to distension of the penis with blood, the reflex impulses resulting from stimulation of the ejaculation centre are transmitted by the motor nerves to certain muscles—those, namely, whose contraction forcibly expels the accumulated semen. The contractions of the affected muscles occur rhythmically, the