434 HISTOLOGY 



infrequently an inverted pyramidal cell is found. The neuraxons and 

 collaterals are medullated. 



The layer oj large pyramidal cells contains those with bodies 20-30 /* 

 long (the "giant pyramidal cells" of the anterior central convolution 

 measure even 80 /*). The very large neuraxon always goes to the white 

 substance, after sending out several collaterals in the gray. 



The layer of polymorphous cells includes oval or polygonal cells which 

 lack a chief dendrite directed toward the surface; their slender neuraxons 

 produce collaterals, and enter the white substance where they may divide 

 into two branches in T-form (Fig. 446, 4). Polymorphous cells with 

 branched neuraxons limited to the vicinity of the cell body, are found in 

 this layer and in the pyramidal layers also. The neuraxon may branch in 

 the molecular layer (Fig. 446, 6). 



Many medullated fibers are found in the deeper layers of pyramidal 

 and polymorphous cells. They are grouped in tapering radial bundles 

 which terminate toward the layer of small pyramidal cells, as seen in 

 Fig. 444. The bundles include the descending medullated neuraxons 

 of the pyramidal and polymorphous cells, and the ascending medullated 

 sensory fibers from the white substance. The latter branch repeatedly, 

 forming the supra-radial and tangential networks. The medullated col- 

 laterals of the pyramidal cells run at right angles with the radial bundles; 

 they form an inter-radial network, the outer part of which is so thick in 

 the region of the calcarine fissure that it can be seen without magnifica- 

 tion, and is there known as the "stripe of Vicq d* Azyr." Similar bands 

 may be detected elsewhere in thick sections (Baillarger's stripes). 



In the gyrus hippocampi and gyrus uncinatus the tangential fibers 

 are so abundant as to form a considerable layer, the substantia reticularis 

 alba. The hippocampus (Ammon's horn), olfactory bulb, and some other 

 areas of the cortex, differ in details from the central region which has been 

 described; these pecularities are considered in the larger special works on 

 the nervous system. 



The neuroglia of the hemispheres, like that of the cord, is at first a 

 syncytium with strands extending from the ventricle to the periphery. 

 Later, the syncytium is divisible into short-rayed neuroglia cells found 

 chiefly in the gray substance, long-rayed cells found chiefly in the white, 

 and ependymal cells lining the ventricles. The ependymal layer is con- 

 tinuous through the aqueduct with that of the fourth ventricle and central 

 canal. In early stages its cells have cilia-like processes which are in part 

 retained in the adult. The short-rayed cells, which are characterized by 

 knotted branching processes, are often in close relation with the blood 

 vessels; they may serve to transfer the nutritive and myelin-forming 

 material from the vessels to the nerve fibers. The outer surface of the 

 cerebral cortex is covered with a feltwork of neuroglia fibers. 



