Prelibido, which is especially characterized by engorgement, swelling, and erection of the corpora cavernosa of the male and female reproductive organs, occurring as a reflex from the spinal cord, may be experienced long before puberty; it is much more independent of processes occurring in the reproductive glands than is the terminal libido, or sexual gratification, which in the male accompanies ejaculation of the semen, and is associated with conditions attained only at puberty. The actual origin of the sexual tension which ultimately leads to ejaculation is still obscure; it seems, at first sight, probable that in the male this sensation is connected with the accumulation of semen in the seminal vesicles. Pressure on the walls of these structures may be supposed to stimulate the sexual centres in the spinal cord, and also those in the brain; but this theory fails to take into account the condition in the child, in woman, and in castrated males, in all of whom, notwithstanding the absence of the accumulation of any reproductive products, nevertheless a distinct state of sexual tension may be observed. It is, indeed, an old experience that eunuchs may have a very powerful sexual impulse. It is obvious, then, that the sexual impulse must be, to a very great extent, independent of the reproductive glands. The nature of sexual tension is still entirely unknown. Freud assumes, in view of the recently recognized significance of the thyroid glands in relation to sexuality, that possibly some substance generally diffused throughout the organism is produced by stimulation of the erogenic zones, that the products of decomposition of this substance exercise a specific stimulus on the reproductive organs, or on the associated sexual centre in the spinal cord. For example, such a transformation of a toxic, chemical stimulus into a special organ-stimulus is known to occur in the case of certain foreign poisonous materials introduced into the body. Freud considers that the probability of this chemical theory of sexual excitement is increased by the fact that the neuroses referable to disturbances of the sexual life possess a great clinical similarity to the phenomena of intoxication induced by the habitual employment of aphrodisiac poisons (certain alkaloids). The relief of sexual tension occurs in the natural way in the =sexual act=, in the completion of normal intercourse between man and woman. Notwithstanding the numerous observations of leading natural philosophers and physicians concerning the act of sexual congress, among which I need only refer to those of Magendie, Johannes Müller, Marshall Hall, Kobelt, Busch, Deslandes, Roubaud, Landois, Theopold, Burdach, and many others, we possess, for reasons it is easy to understand, no really exact investigations regarding the different phenomena occurring during the sexual act. More particularly, the demeanour of the woman during this act is a matter which remains extremely obscure.