_Genital centres._—The centres for the genital nerves are six in number, three cerebral, one in the medulla oblongata and two spinal centres. There is first the cerebral centre of voluptas or cupido. It is the seat of the sexual instinct or of sex desire. The second centre is the centre of libido. It is the centre for experiencing pleasure. The third centre is that of inhibition. It is the centre where, under certain circumstances, sexual activity is prevented. The vasodilator centre in the medulla oblongata and the two spinal centres regulate erection and ejaculation. Connecting fibres are passing between the cerebral centres and the spinal centres. Hence psychic stimulation may cause erection and ejaculation. CHAPTER IV THE FEMALE ORGANS OF GENERATION _Vulva._—The main female generative organs are entirely situated within the pelvis. The less important external organs are those comprised in the name vulva (derived from the Latin word valva, the folding door). When the woman is in erect position, the vulva runs horizontally from the mons veneris to the frenulum of the fossa navicularis. [Illustration: CUT XII. _Vulva, the nymphae separated._ 1, mons veneris; 2, labium majus; 3, prepuce of clitoris; 4, glans of clitoris; 5, urethral orifice; 6, nympha; 7, orifice of the Bartholinian gland; 8, vaginal orifice; 9, hymen; 10, frenulum; 11, perineum; 12, anus.] _Mons veneris._—The mons veneris is the name of the fatty cushion which rests upon the anterior surface of the symphysis pubis. After puberty, the mountain is covered with a growth of hair. In the female sex the hairs occupy a triangular area, the base corresponding with the upper margin of the symphysis. This sign is sometimes of value for the determination of the real sex of an apparent hermaphrodite. _Labia majora._—The labia majora are a pair of integumentary folds, extending from the mons veneris to the perineum, or the triangular partition between the anus and the vagina. Each labium has two surfaces, an outer one pigmented and covered with strong, crisp hair as on the mons veneris, and an inner one, usually lying in close apposition with its fellow. The fissure between the two labia is termed rima pudendi. The inner surfaces are moist and resemble a mucous membrane in appearance. The outer surface has the same structure as the skin. Beneath the skin there is found a layer of connective tissue, rich in elastic fibres and fatty tissue. The next layer is a dense mass of adipose tissue, which is supplied with an abundant plexus of veins. The labia are richly supplied with sebaceous glands. [Illustration: CUT XIII. _Labium majus and sebaceous gland._ g, gland; sc, stratum corneum; e, layers of pavement epithelium; f, fibrous tissue; v, blood vessel.]