1. SYSTEMATIC LESIONS OF THE ÆSTHESODIC SYSTEM OF THE ENCEPHALON.—The limits of this system are as follows: Within the oblongata and pons it occupies a somewhat uncertain (from a clinical standpoint) location, its fasciculi and ganglia lying in a general way dorso-laterad of the motor or kinesodic system. In the crus cerebri the fibres of the sensory tract are more closely packed together, and constitute a dense fasciculus of white substance in the lateral part of the crus in its subcerebral extremity, estimated by Flechsig at about one-fifth of the entire crus, and thence it enters into the composition of what is known as the internal capsule. This narrow but all-important mass of {82} white fibres, as revealed by a horizontal section of the hemisphere (vide Fig. 7), lies between the nucleus caudatus and the nucleus lentiformis, and between the latter and the thalamus, thus assuming a V or elbow shape. Of this, the caudal third of that part of the capsule behind the bend or angle contains all the sensory paths for the opposite half of the body, reinforced by paths for the special senses; it is the carrefour sensitif of French writers. From this region sensory fasciculi radiate to various parts of the cerebral cortex—in the parietal, temporal, and occipital gyri—in which physiological experiments and human cases of disease have shown sensory areas or centres. Of the fasciculi from special sense-organs which reinforce the internal capsule, the only one which is well defined and easily traceable is the fasciculus opticus (fasciculus of Gratiolet), derived in part directly from the optic tract of the same side and from the primary optic centres (lobus opticus, corpus geniculatum laterale, and pulvinar), proceeds, along with fibres of the internal capsule, dorso-laterad of the posterior cornu of the lateral ventricle, to the mesal aspect of the occipital lobe. A fasciculus from the olfactory apparatus doubtless also joins the internal capsule, but its course is unknown. The following localization diagnoses are now possible in the æsthesodic system as above defined: (_a_) A lesion of the internal capsule, just above the crus cerebri, involving its bend or knee and caudal segment, with or without injury to the nucleus lentiformis and thalamus, will give rise to the following symptoms: Complete typical hemiplegia and total hemianæsthesia on the opposite side of the body; the anæsthesia involves the special senses as well as the body. Vision is, however, unaffected, unless the lesion extend far enough back to involve the pulvinar and fasciculus opticus, when lateral hemianopsia occurs (dark half-fields on the side opposite the lesion). When this lesion is in the left hemisphere, sensory aphasia also occurs.