Our knowledge of the psychoses accompanying paralysis agitans is very inadequate. The disease was first fully described by Parkinson in an English publication in 1817, although, according to Camp, similar cases were reported by Schwarz in 1766. The etiology of this condition is unknown and the pathology is not at all definite. It seems to be the rather general opinion of neurologists that mental disturbances are quite rare in Parkinson's Disease. Camp,[199] for instance, has the following to say on this subject:—"Mental conditions have also been described, but usually the patient's mind is entirely clear. In the very old the changes incident to senility, such as irritability, childishness, etc., insomnia and memory changes, might be expected and may require special treatment. Often these patients are emotionally unstable and spells of forced weeping or laughter occur." Krafft-Ebing refers to mental weakness in paralysis agitans and speaks of the frequency of melancholia with hallucinations and suicidal impulses occurring intermittently and appearing with exacerbations of the disease. He speaks of premature senility as playing the most important etiological rôle. McCarthy[200] expresses the opinion that: "Beyond a tendency on the part of some patients to adopt a whining and complaining manner, the mind remains very clear; in fact, the good nature and complaisance of most of the patients, in spite of the severity of the symptoms, is a matter of common observation. Dementia may, however, complicate a case of the disease." On the other hand, Parant, a French writer who made an elaborate study of this subject in 1883, described three distinct varieties of mental disturbance observed by him. In the milder cases he found changes in the personality. This is shown by irritability, egotism, restlessness, suspicion, undue sensitiveness regarding their disease, mild persecutory ideas, tendencies to depression, indifference and apathy. The second class of cases described included mental deterioration with difficulty of thought, loss of memory, etc. The third group includes definite psychoses characterized generally by depressions with or without hallucinations and delusions. Hallucinations of sight are said to be common. Delusions of persecution are prominent and hypochondriacal and somatic ideas frequently occur. Suicidal tendencies are very pronounced. According to Ball, these episodes come and go "with the aggravation of the sensory symptoms, and they seem to disappear when the tremor decreases or ceases entirely." The usual tendency in these cases, as shown by Parant, is to terminate in complete deterioration.