Hereditary transmission is certainly a cause also of many diseases of the nervous system. Friedrich's ataxia is hereditary. It is an incurable progressive incoördination of the limbs, tongue, larynx, and eyes, which attacks commonly between the tenth and the twentieth year, and the patient dies from some intercurrent disease, usually an infection. Progressive muscular dystrophy is also hereditary and incurable. The legs and trunk atrophy, and death comes from an intercurrent disease. Related to this malady are hereditary progressive neurotic muscular atrophy, progressive spinal muscular atrophy in infants, and progressive spinal amyotrophy in adults. Amaurotic (_amaurosis_, blindness) family idiocy is hereditary, and the child dies at about two years of age. Huntington's chorea appears in every generation of an affected family. Its symptoms show between the ages of thirty and forty years, and it progresses from choreic and ataxic signs to dementia and death. The death is often by suicide. In eastern Long Island, southwestern Connecticut, and eastern Massachusetts 962 cases were all traced back to six persons, three of whom were probably brothers, who came to America in the seventeenth century. In the 3000 relatives of these choreics were 39 cases of epilepsy, 51 cases of cerebral inflammation, 41 cases of hydrocephaly, 73 feeble-minded children, and other evidences of neuroses. The heredity in this disease is apparently Mendelian. Besides the diseases enumerated here, there are several pathologic conditions of the eyes which are hereditary--presenile cataract, stationary night blindness, and retinitis pigmentosa. If the persons who have these diseases are sexually sterile, evidently the heredity so far as they are concerned will be cut across; hence the advocates of legal sterilization wish to have these patients sterilized to protect society. The surgical operation by which the man is sterilized according to the State laws mentioned above is an interruption of the continuity of the vasa deferentia near the testicles. This interruption may be a severing of each vas, a cutting out of a part of each vas, or a ligation of the vasa. The term vasectomy is now used loosely to cover all these methods. The vas deferens, or seminal duct, passes from the testicle up along the groin on each side, in through the belly-wall by the inguinal canal, down along the pelvis and under the bladder, where it opens into the bottom of the urethra a short distance in front of the bladder exit. Each vas is about two feet in length, and it has a diameter of one-tenth of an inch throughout the greater part of its length, but its lumen is extremely narrow.