II. PHYSIOLOGY.
During the time of the physiological processes in the reproductive glands, desires arise in the consciousness of the individual which have for their purpose the perpetuation of the species (sexual instinct).
Sexual desire during the years of sexual maturity is a physiological law. The duration of the physiological processes in the sexual organs, as well as the strength of the sexual desire manifested, vary, both in individuals and in races. Race, climate, heredity, and social circumstances have a very decided influence upon it. The greater sensuality of southern races as compared with the sexual needs of those of the North is well known. Sexual development in the inhabitants of tropical climes takes place much earlier than in those of more northern regions. In women of northern countries ovulation, recognizable in the development of the body and the occurrence of a periodical flow of blood from the genitals (menstruation), usually begins about the thirteenth or fifteenth year; in men puberty, recognizable in the deepening of the voice, the appearance of hair on the face and the mons veneris, and the occasional occurrence of pollutions, etc., takes place about the fifteenth year. In the inhabitants of tropical countries, however, sexual development takes place several years earlier in women,—sometimes as early as the eighth year.
It is worthy of remark that girls who live in cities develop about a year earlier than girls living in the country, and that the larger the town the earlier, ceteris paribus, the development takes place.
Heredity, however, has no small influence on libido and sexual power. Thus there are families in which, with great physical strength and longevity, great libido and virility are preserved until a great age, while in other families the vita sexualis develops late and is early extinguished.
In women the time of the activity of the reproductive glands is shorter than in men, in whom the sexual function may last until a great age. Ovulation ceases about thirty years after puberty. This period of cessation of activity of the ovaries is called the change of life (climacterium). This biological phase does not represent merely a cessation of function and final atrophy of the reproductive organs, but also a transformation of the whole organism. In Middle Europe the sexual maturity of men begins about the eighteenth year, and their virility reaches its acme at forty. After that age it slowly declines.
The potentia generandi ceases usually at the age of sixty-two, but potentia cœundi may be present even in old age. The existence of the