The ‘primary registration’ of ‘common sensation’ occurs in the post-central gyrus, immediately posterior to the fissure of Rolando. This tactile area occupies a position behind the fissure of Rolando similar in extent to that occupied by the motor area in front. It commences at the bottom of the fissure of Rolando and extends backwards over rather more than half the exposed area of the post-central convolution. It reaches down to near the Sylvian fissure and extends over on to the mesial aspect of the brain. Furthermore, it is probable that sensation in any given part lies on more or less the same level as the corresponding motor area.
Immediately posterior to the tactile area and occupying the posterior and upper part of the post-central convolution, the area responsible for muscle-sense is situated.
Stereognosis—memory pictures, object perception, &c.—is referred to the superior parietal lobe.
Primary visual impressions are received in the occipital lobe, more especially on the mesial aspect thereof.
Finally, the four areas concerned in speech—motor speech, writing, reading, and hearing—are anatomically separated from one another and yet closely associated, so much so that one can hardly be involved without the other. The motor speech centre of Broca has already been mentioned as occupying—in right-handed individuals—the posterior part of the third left frontal convolution. Writing lies immediately above and in front, in the posterior part of the middle frontal gyrus, auditory impressions are received in the posterior and upper part of the first temporo-sphenoidal lobe, whilst the power of reading is dependent on the integrity of the supramarginal and angular gyri (see also [Fig. 57]).
Smell and Taste lie in close relation to the anterior pole of the temporo-sphenoidal lobes.
[1] Reid’s base-line is a line drawn around the skull, starting in front at the lower border of the orbit and passing through the central point of the external auditory meatus.
[2] Proceedings and Philosophic Transactions of the Roy. Soc., 1901.
[3] Localization of Cerebral Function. Camb. Univ. Press, 1905.