Origin and purpose of the reproductive organs -- Progressive differentiation of these organs -- Original identity of their rudiments in the two sexes -- Weininger’s theory of the intermixture of the sexual elements -- This theory anticipated by Heinse -- Bisexuality -- The actual significance of bisexuality trifling -- Phylogenetic explanation of the organs of sexual congress -- Bölsche’s three problems -- The “aperture-problem” -- Connexion between the genital aperture and the urinary passage -- Between the genital aperture and the anus -- Significance in relation to certain sexual aberrations -- The “member-problem” -- Earlier modes of fixation during coitus -- Sucking and biting -- The action of the limbs (the embrace) -- The penis -- Its various forms -- The penis-bone -- The free character of the human penis -- The descent of the testicles -- The feminine rudiment of the penis -- Its original function rendered superfluous by the further evolution of the sexual orifice -- Transformation into the clitoris and the labia minora -- The “libido-problem” -- Voluptuousness a phenomenon of distance-love -- Questionable specificity of voluptuousness -- Theory of the “sexual sense” and of the “sexual cells” -- Relations of voluptuousness to tickling and to painful sensations -- A special variety of contact stimuli -- Localization to the genital organs -- The sexual impulse -- Relative independence of the impulse from the reproductive glands -- Genesis of sexual excitement -- Stage of prelibido (sexual tension) -- Terminal libido (sexual gratification) -- Symptoms and early appearance of prelibido -- Causes of sexual tension -- Freud’s chemical theory of sexual tension -- The act of sexual intercourse -- Roubaud’s description of coitus -- Demeanour of woman in coitus -- Magendie on this subject -- Dr. Theopold’s observations -- Physiological phenomena associated with coitus -- Sadistic and masochistic elements -- The normal position during sexual intercourse -- Figuræ Veneris -- Significance of the normal position in relation to civilization. CHAPTER III