These details have been explained in order that the young man may fully understand the physiology of his sexual apparatus and not be disturbed by the advertisements or the pamphlet literature of charlatans who make a business of frightening young men into the belief that in these experiences they are losing "vital fluid"--that they are victims of "lost manhood," or that they are entering into a condition of "general debility" and "impotence." As an actual fact, involuntary loss of vital fluid (spermatorrhea), is a rare case even in the practice of specialists in genito-urinary diseases, and in these rare cases, the condition is usually a result of very great excesses, sexual debauchery or one of the sequelæ of venereal disease. [Read: Appendix 1, 13 and 17.] e. =Testes.= No rational idea of the physiology of the testes can be given without laying down as a fundamental physiological law, that _the testes secrete under sexual stimulation only_. This same general principle applies to all glands, i.e., that they secrete only under the influence of some special stimulation. In harmony with that law, the testes secrete only under the influence of sexual stimulation. The sexual stimulation may be sub-divided into two general categories, i.e., conscious sexual stimulation and subconscious sexual stimulation. _Conscious sexual stimulation_ is partly psychical and partly physical. The physical stimulation is produced by physical proximity of a member of the opposite sex. The physical and psychical phases of conscious sexual stimulation are so intimately interwoven that it is exceedingly difficult to discuss one without constant reference to the other, and it may be said in this connection that the psychical attitude of the two individuals of opposite sex who are brought into close physical proximity will modify very greatly their local sexual responses. Reverting to the lower animals: when a female in rut or heat is brought into proximity to the male, there seems to be on the part of each animal a consciousness of the character and attitude of the other animal and both animals are step by step excited by various physical contacts and probably also psychical conditions to a high state of sexual excitement, leading to the natural ultimate result, coitus, in which event the sexual excitement culminates in the orgasm of the male, which empties the secreted semen into the organs of the female. It will be easily understood that, in human subjects whose social relations permit them to indulge in coitus, close physical proximity, and various caresses lead, step by step in the normal course of nature to sexual excitement and sexual desire which culminates as described above for the lower animals.