The _naturæ frigidas_ of Zacchias are examples of a milder form of anæsthesia. They are met more frequently among women than among men. The characteristic signs of this anomaly are: slight inclination to sexual intercourse, or pronounced disinclination to coitus without sexual equivalent, and failure of corresponding psychical, pleasurable excitation during coitus, which is indulged in simply from sense of duty. I have often had occasion to hear complaints from husbands about this. In such cases the wives have always proved to be neuropathic _ab origine_. Some were at the same time hysterical. _2. Acquired Anæsthesia._ Acquired diminution of sexual instinct, extending through all degrees to extinction, may depend on various causes. These may be organic and functional, psychical and somatic, central and peripheral. The diminution of libido, as age advances, and its temporary disappearance after the sexual act, are physiological. The variations with reference to the duration of the sexual instinct are dependent upon individual factors. Education and manner of life have a great influence upon the intensity of the vita sexualis. Intense mental activity (hard study), physical exertion, emotional depression, and sexual continence decidedly diminish sexual inclination. Continence at first induces increase, but sooner or later, according to constitutional conditions, the activity of the generative organs decreases, and with it libido. At all events, in a person sexually mature, a close connection exists between the activity of the generative glands and the degree of libido. That this relation is not determinate is shown by the cases of sensual women, who, after the climacterium, continue to have sexual intercourse, and may manifest states of sexual excitement (cerebral). Also in eunuchs it is seen that libido may long outlast the production of semen. On the other hand, however, experience teaches that libido is essentially conditioned by the function of the generative glands, and that the facts mentioned are exceptional manifestations. As peripheral causes of diminution or extinction of libido, may be mentioned castration, degeneration of the sexual glands, marasmus, sexual excesses in the form of coitus and masturbation, and alcoholism [cocainism]. In the same way, the disappearance of libido in general disturbances of nutrition (diabetes, morphinism, etc.) may be explained. Finally, the atrophy of the testicles should be remembered, which has sometimes been observed to follow focal lesions of the brain (cerebellum). A diminution of the vita sexualis, from degeneration of the tracts of the cord and genito-spinal centre, occurs in diseases of the spinal cord and brain. A central interference with the sexual instinct may be organically induced by cortical disease (dementia paralytica in its advanced stages); functionally, by hysteria (central anæsthesia?) and emotional insanity (melancholia, hypochondria).