A unilateral lesion of the _spinal cord_ causes a lower neurone paralysis of the muscles supplied from the cord at the level of the lesion, with spastic paralysis of the muscles of the same side of the body supplied from a lower level of the cord. The sensory symptoms are variable. Typically there is some anæsthesia in the structures supplied from the damaged section of the cord--incomplete owing to the overlapping by other sensory nerves. Just above the lesion there is irritation of spinal nerves, and hyperæsthesia and pain referred to their distribution. On the same side below the lesion, there is a loss of epicritic, stereognostic and deep sensibility, and on the opposite side below the lesion, loss of the sense of pain and the discrimination between heat and cold. Ordinary tactile sensibility, which is governed by a double path, may or may not be lost on either side below the lesion. #Other Special Centres.#--The cortical centres for _vision_ lie on the median surfaces of the occipital lobes in the neighbourhood of the calcarine fissure. Each half-vision centre--for there is one in each occipital lobe--receives the fibres from the same side of both retinæ. Destruction of one half-vision centre produces the condition known as _homonymous hemianopia_, in which the medial (nasal) half of one visual field and the lateral (temporal) half of the other is affected, so that there is an inability to see objects situated on the side opposite to the lesion. _Auditory impulses_ are received in the posterior part of the superior temporal convolution. _Aphasia._--The use of language, spoken or written, as a means of expression depends upon the co-ordination of four different centres: the visual, the auditory, the graphic, and the articulatory. These are situated in different parts of the brain and are connected by sub-cortical association tracts, the main pathway of which lies in the vicinity of the upper end of the fissure of Sylvius. Marie has proved that aphasia results from lesions in this area. The _olfactory_ and _gustatory_ centres are situated in the uncus close to the pituitary fossa.