2. _Anæsthesia_ (absence of sexual instinct). Here all organic impulses arising in the sexual organs, as well as all concepts, and visual, auditory, and olfactory sense-impressions, fail to excite the individual sexually. This is physiological in childhood and old age. 3. _Hyperæsthesia_ (increased desire, satyriasis). In this state there is an abnormally increased impressionability of the vita sexualis to organic, psychical, and sensory stimuli (abnormally intense libido, lustfulness, lasciviousness). The stimulus may be central (nymphomania, satyriasis) or peripheral, functional or organic. 4. _Paræsthesia_, (perversion of the sexual instinct, _i.e._, excitability of the sexual functions to inadequate stimuli). These cerebral anomalies fall within the domain of psychopathology. The spinal and peripheral anomalies may occur in combination with them, but these affect persons, as a rule, that are free from mental disease. They may occur in various combinations, and become the cause of sexual crimes. For this reason, they demand consideration in the following description. However, the cerebral anomalies claim the principal interest, since they very frequently lead to the commission of perverse and even criminal acts. A. PARADOXIA. SEXUAL INSTINCT MANIFESTING ITSELF INDEPENDENTLY OF PHYSIOLOGICAL PROCESSES. _1. Sexual Instinct Manifested in Childhood._ Every physician conversant with nervous affections and diseases incident to childhood is aware of the fact that manifestation of sexual instinct may occur in very young children. The observations of Ultzmann concerning masturbation in childhood[31] are worthy of attention in relation to it. It is necessary here to differentiate between the numerous cases where, as a result of phimosis, balanitis, or oxyuris in rectum or vagina, young children have itching of the genitals, and experience a kind of pleasurable sensation from manipulations thus induced, and thus come to practice masturbation; and those cases in which sexual ideas and impulses occur in the child as a result of cerebral processes without peripheral causes. It is only in this latter class of cases that we have to do with the early manifestation of sexual instinct. In such cases it may always be regarded as an accompanying symptom of a neuro-psychopathic constitutional condition. A case of Marc’s (“Die Geisteskrankheiten,” etc., von Ideler, i, p. 66) illustrates very well these conditions. The subject was a girl of eight years, of respectable family, who was devoid of all child-like and moral feelings, and had masturbated from her fourth year; at the same time she consorted with boys of the age of ten or twelve. She had thought of killing her parents, that she might become her own mistress and give herself up to pleasure with men. In these cases of early manifestation of libido the children come also to masturbate; and, since they are