This farthest point is an =act of contact=.[16] We have already learnt to regard the skin as a projection of the nervous system, and we have come to understand the significance of the skin in the sphere of sexuality. The other senses which have arisen from the skin must also be taken into account in this matter. In the genital organs, this touch stimulus assumes a quite peculiar character; it gives rise here to the proper voluptuous sensation which is associated with the discharge of the reproductive products. In man this association is most distinctly manifest. The instant of most intense sexual pleasure coincides with ejaculation, with the expulsion of the semen. The character of this voluptuous sensation can hardly be defined; in part, it is like an intense tickling sensation, but, on the other hand, it has an unmistakable relationship to pain. Later, in another connexion, we shall consider this interesting point at greater length. Not inaptly the sexual act has also been compared with sneezing; the preliminary tickling sensation, with the subsequent discharge of nervous tension, in the form of a sneeze, have, in fact, a notable similarity with the processes occurring in the sexual act. The sexual act depends upon the occurrence of certain stimuli which are connected with the complete development of the internal and external genital organs and of the reproductive glands. The time when this development occurs in man and woman is known as =puberty=. The sum of these stimuli is known as the “=sexual impulse=.” Whereas in the lower animals the sexual impulse is for the most part connected with the activity of the reproductive glands, in the human species, in association with the preponderating significance of the brain, it has attained a relative independence of the reproductive glands; whilst the mind has come to influence the sexual impulse very powerfully. Generally speaking, sexual excitement is produced in three ways: first, by the activity of the reproductive glands; secondly, by peripheral excitement derived from the so-called “erogenic” areas; and thirdly, by central psychical influences. S. Freud has recently studied the relations between these three causes of sexual excitement, of the sexual impulse, and has very properly distinguished two stages--the stage of “=prelibido=” (sexual desire), and the stage of the proper sexual “=libido=” (sexual gratification). The stage of prelibido has distinctly the character of tension; the stage of libido, the character of relief. The feeling of tension during the prelibido finds expression mentally as well as physically by a series of changes in the genital organs. The tension is further increased by the stimulation of the various erogenic zones. If this prelibido increases beyond a certain degree, the characteristic potential energy of sexual tension is transformed into the relief-giving kinetic energy of the terminal libido, during which the evacuation of the reproductive products occurs.