=Greatest Growth= is between the first appearance of the primitive brain and the end of the sixth month of life (extra-uterine), hence it is during this period that any adverse conditions relative to development of nerve cells may cause the greatest damage. =Mind and Brain.=—Whatever may be the connection between these two, we know that the former develops with the growth of brain cells and fails with their decay. =Amentia= is associated with the incomplete development of brain cells and =Dementia= is coincident with their degeneration and death. =Pathology=—=Brain.=—Structural abnormality of the brain tissue may exist without variation of mentality or defect. Early observers gave these gross defects as a cause for amentia. However, it has been demonstrated beyond doubt by microscopic examination of cerebral neurosis that cellular changes occur and that imperfect and arrested development exists and is an essential basis of amentia. =Histology=—=Blood Cells.=—Cortical blood cells in the ament are 1. Numerically fewer. 2. Irregular in arrangement. 3. Imperfectly developed. 4. Microscope reveals changes proportionate to the deficiency during life. =Blood-vessels in Amentia= show no marked changes from those of the normal brain. Hyaline degeneration may be present; also pigmentation. These conditions are not constant in amentia hence cannot be considered causal. =Neuroglia in Amentia.=—Sclerosis and hypertrophy occur in a large proportion of cases. This is diffuse throughout the brain, with here and there certain circumscribed areas forming nodules. =Nerve Fibres of Cortex in Amentia.=—Association system fibres are always diminished in number and not so complicated. =Clinical Varieties of Amentia.=—There are two varieties of amentia and conventionally for sake of study we must arrange them into those from (1) Congenital causes and (2) acquired causes. Among those which arise from congenital causes we have the microcephalous and Mongolian types. In both cases there exist constitutional taints through successive or immediately forgoing generations of such diseases as syphilis, tuberculosis, epilepsy, and acute alcoholism affecting proper collaboration of germ cells previous to fertilization and hence impaired germinal endowment through a weakened nervous system. Those arising from acquired causes are from injury to mother or fetus. =Macrocephalus.=—A person whose skull measures less than seventeen inches in its greatest circumference. This class comprises less than 10% of all aments. =Cause.=—The type is neither a freak reversion of the species to a lower grade of development nor accidental, but due to an inherited blight on the nervous system arising from constitutional disease, alcoholic and sexual excesses, consanguinous unions and too numerous latter-life pregnancies in undermined health states. They come entirely from neuropathic stock and their brothers and sisters are degenerates. Many dwarfs exhibit this type.