THE SENSORY CENTRES.
a. The sensory area, including the centres for touch, pain and temperate sensi bility, embraces especially the postcentral convolution and the adjoining anterior part of the parietal lobe and, perhaps, even extends onto the precentral convolution. The position- and movement-sensibility, as well as space- and orientation-sense, are trans ferred to this same region. The impulses, passing to the sensory area, come essentially from the opposite half of the body.
b. The auditory centre is located in the middle part of the gyrus temporalis superior and includes additionally the gyri transversi of the upper temporal convolution, that lie buried within the fissure cerebri lateralis.
c. The visual centre lies within the cuneus, par ticularly within the cortex of the fissura calcarina, perhaps extending onto the gyrus lingualis.
d. The olfactory cen tre is situated within the anterior part of the gyms hippocampi and the hippo campus.
e. The gustatory cen tre has as yet not been definitely located, but probably adjoins that for smell.
The motor centres and the individual sensory regions or sense-centres are designated also as projection centres, for the reason that the impulses passing from the stimulus-receiving organs (skin, muscles, joints and the higher sense organs), and conveyed by the sensory nerves of the central nervous system, are radiated or projected, as it were, to the sense-centres, while the impulses from the motor centres are similarly projected towards the periphery and conveyed by the motor nerves especially to the muscles. Stimulation within the sense-centres induces sensation (touch, sight, hearing, smell, etc.) ; stimulation within the motor zone leads to movements. These incoming and outgoing conductions follow quite definite paths, which are termed afferent or centripetal and efferent or centrifugal projection tracts respectively.
Viewing the surface of the cerebral hemispheres and imagining the individual pro jection centres outlined, it will be evident that the latter occupy only a certain part, perhaps a third, of the entire cerebral cortex. In addition to these motor and sensory fields, there remains a large area that embraces certain parts of the frontal, parietal, occipital and temporal lobes, together with the deeply situated central lobe or the insula—a tract of still slightly known function. According to Flechsig, this entire large area is designated the association centres, of which, following him, an anterior, a middle and a posterior association centre are distinguished. The anterior or frontal centre in
cludes the fore-part of the frontal lobe; the middle or insular centre, the island of Reil; and the posterior or parieto-occipito-temporal centre, a large part of the occipital and temporal lobes and almost the entire parietal lobe.
According to the theory advanced first by Flechsig, these association areas constitute the substratum for the higher psychic functions—an apparatus which collects the activities of the sense-centres to higher unity, and comprises centres of all the more complex associations. They are the chief bearers of what we call experience, knowledge and cognizance and, in part, of speech, in short the intellectual centres proper. Flechsig was led to the advancement of this theory chiefly through his histological investigations based on the method of the development of the medullary sheath. He proved that the myelin-ripening of the individual nerve-tracts proceeds from below upward, that is from the spinal cord and the lower brain-segments towards the cortex of the end-brain. Already at birth, according to Flechsig, the individual tracts have reached, in large part, their development within the lower divisions of the brain, while within the cerebrum only few paths of conduction have developed. At first, one sense-conductor after the other gradually pushes out towards the cerebral cortex. In the new-born child, only two of the sense-centres, the olfactory and gustatory, are developed ; then follow the centres for tactile sense, for sight and, lastly, for hearing. Only subsequent to the com pleted development of the sense-centres, does the development of the intellectual centres begin within the individual territories. Medullary fibres proceed from the projection centres to the neighboring association areas, the latter likewise become functionally active and eventually numerous tracts bind both kinds of centres with each other. Based on further investigations, Flechsig later subdivided the entire cerebral cortex into thirty-six different areas, according to the time of completed myelination. The areas first be coming medullated correspond to the projection centres ; then follow the embryonic intermediate centres and, finally, the terminal districts, which exclusively form the association centres.