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Association Fibers

layer, gyrus, short, cortex, axones, centers and motor

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ASSOCIATION FIBERS These fibers remain on the same side and connect parts of the same hemisphere. They are situated within or beneath the cor tex, the various parts of which they serve to unite. Associa tion fibers become medullated and actively functional only as mental effort and education gradually develop them. So far as the brain is concerned education consists, first, in the develop ment of the functional centers of the brain; and, second, in the establishment of lines of rapid communication between them.

The short association fibers are the more numerous and are very important. They unite contiguous parts of the same gy rus and associate together adjacent gyri. They are intralobar.

In direction they comprise arcuate and tangential fibers; and they are intracortical and subcortical, in position. Every zone of the cerebral cortex contains association fibers, from the felt work of Kaes to the stratum zonale. But they are found chiefly (r) in the radiary zone and adjacent part of the supraradiary zone, along the lines of Baillarger (Fig. 8o); and (2) in the zonal layer (Figs. 84 and 85). The deeper of these fibers are con tinuous with the radiations of Meynert and probably do not belong to the short association fibers, if associative at all (they are corticipetal fibers); the more superficial intersect the ra diations at right angles and are truly associative in function.

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The associative fibers of Meynert are compacted together by pressure in the walls and floor of the sulci. In the crown of a gyrus they are scattered. Their exact origins are not yet worked out; but they are probably the horizontal processes of cells in the second to sixth layers. (2) The association fibers of the plexiform layer of the cortex, which constitute the stratum zonale (Fig. 84), are quite short; they join together immediately contiguous parts within circumscribed areas. The richness of the zonal layer of fibers, as already pointed out in describing the plexiform layer of the cortex, varies greatly in different regions, being best developed in the subiculum. The fibers comprising the zonal layer have four sources of origin: (a) The axones and dendrites of the cells of Golgi and Cajal in the plexi form layer. (b) The apical dendrites of the subjacent pyra

mids. (c) The T-branched axones of Martinotti's cells. (d) The corticipetal axones which terminate in the superficial layer of the cortex.

The short association fibers are almost infinite in their con nections. They connect the receptive and psychic sensory areas, and their interruption on the left side causes inability to interpret the sensations, agnosia, called mind-blindness, mind deafness, stereagnosis, etc. Again, those short fibers also associate the psychic with the psychic-motor, and the psychic motor with the emissive-motor. centers. In this manner the writing center is connected with the motor center for the upper extremity, and the speech center with the motor centers for the lips, tongue, etc.: breaking of the former connection on the left side destroys ability to write, agraphia; and aphasia results, if the latter connection is broken. Besides these and many other connections of associated centers, the short fibers join together the various parts of each cortical area.

The long association fibers (Figs. 97—roo) are collected into bundles. They rise from the pyramidal, the polymorphous and the fusiform layers of the cerebral cortex (Cajal), and are axones. Proceeding out of the lobe in which they rise, being interlobar, they dip down into the centrum semiovale and arbor ize about neurones in more or less distant parts of the cortex... Among the best known are the following bundles: 1. The cingulum of the gyrus fornicatus (Fig. 98) is a bundle of fibers in that gyrus which almost entirely encircles the corpus callosum. It extends from the anterior perforated substance through the gyrus cinguli and hippocampal gyrus, to the uncus and temporal pole. The fibers, which form several systems, radiate from the limbic lobe to the surrounding gyri of the medial surface; they join the limbic lobe with the superior frontal, the paracentral, the precuneate, the cuneate, the lingual and the fusiform gyri. The cingulum does not form a con tinuous strand through the gyrus fornicatus; hence, the name fornix periphericus, given it by Arnold, is not entirely appropriate.

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