As regards the seat of the lesion in common hemiplegia, it may here be said, in general terms, that it is in the cerebral hemisphere opposite to the paralysis (with excessively rare exceptions which are susceptible of explanation), in its motor cortex, in the subjacent associated white fasciculi, or in the knee and caudal part of the internal capsule; the lesion may directly injure those parts or act upon them by compression. (_β_) Crossed Hemiplegia (hémiplégia alterne).—In this form there is paralysis of many muscular groups on one side of the body, while the facial nerve or some other cranial nerve (or several cranial nerves together) show loss of innervation on the other side of the median line. Theoretically, therefore, there may be as many varieties of crossed hemiplegia as there are cranial nerves, but, practically, we meet only with a few forms, of which the following are the most common: (1) motor oculi (N. iii.) on one side, and body and face on the other; (2) facial nerve (N. vii.) on one side, and body on the other; (3) trigeminus nerve (N. v.) on one side (anæsthesia of face, paralysis of masticatory muscles), and body on the other; (4) abducens (N. vi.), facial (N. vii.), and acoustic (N. viii.) together on one side, and the body on the other. (5) With symptoms of No. i. we may have lateral hemianopia, dark half-fields on the same side as paralyzed extremities. In crossed hemiplegias anæsthesia is more common; there is a strong tendency to bilateral extension of the paralysis, and neuro-retinitis is seldom absent before the close of life. As regards the location of the lesion in crossed hemiplegias, it may be stated, in a general way, that it is in the base of the brain on one side of the median line, so placed as to directly injure one or more cranial nerves at their origin, and to compress or destroy the cerebral motor tract (pyramidal tract) above its decussation-point, and in some cases also the sensory tract in the crura, pons, and oblongata.[4] [Footnote 4: For a statement of the exact seat of the lesion causing various forms of crossed hemiplegia, vide article on the LOCALIZATIONS OF LESIONS IN THE NERVOUS SYSTEM.] (_b_) Spinal Hemiplegia.—In this type the face and head are normal, excepting in some cases the iris; the extremities and trunk are more or less paralyzed on one side, the loss of power being more evenly distributed (_i.e._ less distal) than in hemiplegia of cerebral origin. Often there is also anæsthesia, and this is always on the other side of the median line, involving more or less of the whole side. The coincidence of these symptoms below the head indicates positively that the lesion is in the spinal cord, involving one of its lateral halves. Where there is no anæsthesia, care must be taken not to confound the condition with that in which a cerebral lesion causes paralysis of one arm and leg (combined brachial and crural monoplegia).