Two hypotheses, the vaso-motor and the dynamic, chiefly hold sway. The vaso-motor, attractive because of its apparent simplicity, has been well set forth by Walton,[24] who contends that while it may not be competent to easily explain all hysterical symptoms, it will best explain some of the major manifestations of hysteria—for example, hemianæsthesia. Hemianæsthesia, he argues, may appear and disappear suddenly; it may be transferred from one side of the body to another in a few seconds; so blood-vessels can dilate as in a blush, or contract as in the pallor of fear, in an instant. In fainting the higher cerebral functions are suspended, presumably because of vaso-motor changes; therefore the sudden loss of function of one-half of the brain-centres, seen sometimes in hysterical hemiplegia and hemianæsthesia, may easily be imagined to be the result of an instantaneous and more or less complete contraction of cortical blood-vessels on that side. Neurotic patients have a peculiarly irritable vaso-motor nervous system. He records a case seen in consultation with H. W. Bradford. The patient had a right-sided hemianæsthesia, including the special senses, the sight in the left eye being almost wanting. The fundus of the right eye was normal; the left showed an extreme contractility of the retinal blood-vessels under ophthalmoscopic examination; these contracted to one-third their calibre, and the patient was unable to have the examination continued. The explanation offered is, that spasm of the blood-vessels on the surface of the left cerebral hemisphere had caused, by modification of the cortical cells, a right-sided hemianæsthesia, including the sight, and by reaching the meninges a left-sided spastic migraine, and by extending to the fundus of the left eye an intermittent retinal ischæmia. [Footnote 24: _Journal of Nervous and Mental Disease_, vol. xi., July, 1884, p. 424 _et seq._]