236 THE CEREBRUM. 



The commissura hippocampi, the lyre (Fig. 36), unites the 

 hippocampal gyrus, dentate fascia, and the hippocampus with 

 their fellows of the opposite side. This is the commissure of 

 the pyraform lobes, the cortical areas of smell. 



III. ASSOCIATION FIBERS. 



These fibers remain on the same side and connect parts of the 

 same hemisphere. They are situated within or beneath the 

 cortex, the various parts of which they serve to unite. Association 

 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 development 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 gyrus and 

 associate together adjacent gyri. They are intralobar. In direction 

 they comprise arcuate and tangential fibers ; and they are intracort- 

 ical 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 (i) in the radiary 

 zone and adjacent part of the supraradiary zone, along the line 

 of Baillarger (Fig. 60); and (2) in the zonal layer (Figs. 58 and 

 61). (i) Those sparsely scattered large fibers whose location is 

 indicated by the line of Baillarger are called Meynerfs association 

 fibers. The deeper of these fibers are continuous with the radia- 

 tions of Meynert and probably do not belong to the short asso- 

 ciation fibers, if associative at all (they are corticipetal fibers); 

 the more superficial, intersect the radiations at right angles and 

 are truly associative in function. The associative fibers of Mey- 

 nert 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. 61), are quite short when 

 compared with those of Meynert; they join together immediately 



