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What then do these authors mean by a motor image? An auditory or visual image is a mnemonic representation of impressions of sound or light which constitute perception; the auditory and visual images of the words are localized in autonomous centres which are connected with the external world only by a single line in a centripetal direction: these are only centres of impression. But Broca's convolution and the foot of the second frontal, which are the centres of speech and writing, are motor centres, centres of expression, connected with the external world by a centrifugal line, and they are also centres of impression in part and hence connected with the external world by' a second, centripetal line. In Charcot's diagram, the centre of articulate language is connected by this double line with the mouth which pronounces the word (see fig. 16, P: 151).