(_e_) Alterations of sensibility occurring in well-defined areas of the hand, trunk, or extremities, corresponding to the known distribution of nerves, almost always indicates disease of the nerve itself, much more rarely disease in the spinal cord at the origin of the nerve. The reflexes are then diminished or lost. In judging such cases Van der Kolk's law of the distribution of the motor and sensory filaments of nerve-trunks should be remembered: it is, that of the two sets of fibres in a mixed nerve, the sensory fibres are distributed to parts which are moved by muscles which receive the motor fibres of the same nerve. Thus, in nerve lesions the chief sensory symptoms are always peripheral or distal from the chief motor symptoms. (_f_) Disorders of sensibility sometimes appear in patches or irregular areas whose nervous connections are indefinite. Such patches of anæsthesia, hyperæsthesia, or paræsthesia sometimes indicate foci of disease in the spinal cord (and brain?); as, for example, the pains, etc. of posterior spinal sclerosis. These patches may also occur in consequence of {39} interference with local circulation of peripheral parts; and we meet with them in such conditions as hysteria, neurasthenia, alcoholism, etc. (_g_) Universal hyperæsthesia, paræsthesia, and anæsthesia may be observed. The last condition, in the insane, is very apt to give rise to delusions of non-identity and death of the body. THE SENSORY DISTURBANCES OF THE SPECIAL SENSES are well worth separate consideration. (_a_) Optic Apparatus.—Hyperæsthesia of the retina shows itself directly in photophobia, and indirectly (reflexly) by lachrymation and involuntary closure of the eyelids. Paræsthesiæ of the optic nerve and retina show themselves as flashes or projections of white or colored light in the visual field. These may be irregularly or generally distributed in the field, or appear as hemiopic (vide infra), or sector-like forms. Phenomena of this order may be experimentally produced by pressure on the eyeball or by the application of electricity over or near the eye. Photopsiæ of most varied sorts, as flashes, colored scotomata, or quasi forms may immediately precede epileptic seizures or attacks of migraine, constituting an optic aura. In some cases this assumes a definite picture form, when it partakes of the character of an hallucination. Anæsthesia of the optic nerve and retina varies infinitely in degree, from slight blurring or veiling of vision (amblyopia) to complete blindness (amaurosis). Another result is sluggishness or complete immobility of the iris under the action of light. As regards distribution, optic anæsthesia may affect the visual fields uniformly and generally, or it may assume definite geometric forms, or may appear in irregular patches (scotomata).