In the ordinary production of visual sensation, several distinct processes in the human organism are involved. In the retina the ether vibrations (which we know to be still ether vibrations when they reach this surface) are transformed into some other form of energy which can be conveyed along the nerves-we know not what form, but at least it must be something very different from light, because vibrations of that degree of rapidity would cause the destruction of delicate nervous tissues. In the occipital lobes of the cortex there takes place, under the influence of this conveyed excitation, some process which is the immediate condition of the visual sensation. Before reaching the cortex, the optic fibres pass through intermediate ganglionic stations (quadrigeminal bodies, optic thalamus), but it is not known that these have any essential part to play in the sensation that enters consciousness-they may have no other function than to effect reflexly the motions of pupil, ciliary muscle (accommodation), convergence, etc., which are essential to effective vision (Fig. i). When the cortical centres have been destroyed, no visual sensation is possible,

but the same thing is not true concerning the retina: the basal ganglia and the retina may both be thrown out of action by disease, and sensation may nevertheless persist; as a preceding symptom of migraine, which seems to be due to a spasm in the cerebral, or more rarely the retinal, circulation, and of epilepsy, there are very commonly experienced subjective visual sensations, which are sometimes in the form of rings and balls, like the pressure-phosphenes, or zigzags in incompleted curves

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(fortification-figures, scintillating scotomata), but which sometimes have the appearance of natural objects or of human figures. These frequently enter the field of vision at one side, and the patient instinctively turns the head and the eyes to follow them : this shows that the cortical process carries with it what is essential to spatial localization without the participation of the retina. But it also shows, as was plainly affirmed by Gowers before the recent work of Flechsig on the subject, that there are secondary cortical centres (association centres,

5 or, as they may perhaps be designated, perception centres) where the immediate data of visual sensation are worked up into complicated forms. This proves that chemical changes in the cortex, although not brought about by excitation coming in from below, suffice to affect consciousness (and with spatial attribute as well as simple sensation quality). On the other hand, there are cases on record of most disturbing visual sensations (rings and balls of colour) due to irritation of the cortex caused by a diseased retina which was entirely blind to light-as was proved by the fact that these disturbances ceased when the eye in question was enucleated.