THE XERVE SYSTEM 



coordinating fibre systems are as truly representative of the complexity of man's thought appa- 

 ratus as the number of interconnecting wires within a telephone " central" station is indicative 

 of the amplitude of connections possible in that system. The proportions of gray and white 

 substances are expressed in the following tabulation: 



^ f Cortex 33 per cent. 



Gray substance \ , ,. 



J \ Ganglia b per cent. 



White substance 61 per cent. 



The removal, by successive slices, of the dorsal parts of the cerebrum soon 

 brings into view the large expanse of transverse myelinic fibres, the corpus callosum, 

 which connects the two hemispheres. 



The Corpus Callosum (trabs cerebri; commissura maxima). The corpus callosum 

 is a thick stratum of transversely directed nerve fibres, by which almost every part 

 of one cerebral hemisphere is connected with the corresponding part of the other 

 cerebral hemisphere (Figs. 685 and 686). The axones composing it arise from the 



FIG. 685. Diagram of coronal section of cerebrum to 

 show course of fibres of corpus callosum. (Testut.) 



FIG. 686. Diagram of horizontal section of cere- 

 brum to show course of fibres of corpus callosum. 

 (Testut.) 



small pyramidal or the polymorphous cells of the cerebral cortex, or they may 

 be collaterals from the long association or even the projection neurones. They 

 pass in both directions and within the centrum semiovale radiate in various direc- 

 tions (radiatio callosi) between the fibres of the corona radiata to terminate in 

 the layer of small pyramidal cells of the cortex, thus forming a great transverse 

 commissural system, and at the same time roofing in the greater part of the lateral 

 ventricle in each half. A portion of the dorsal surface is free for a width of about 

 1 cm. on either side of the mesal plane, partly covered by the indusium and 

 overlapped by the callosal gyres of the two sides, a fold of pia intervening. 



The mass of radiating fibres may, for convenience, of description, be sub- 

 divided into a pars frontalis, a pars parietalis, and a pars occipitotemporalis. The 

 frontal and occiptotemporal portions are compressed or thickened mesally because 

 the fibres cannot pass directly across, but curve, respectively, frontad and caudad 

 in each hemicerebrum to form two tong-like bundles, the forceps anterior s. minor 

 (preforceps), and forceps posterior s. major (post forceps). The pars parietalis 

 constitutes the greater part of the "body" of the corpus callosum. The fibres 

 traversing the body (truncus corporis callosi) and the adjacent part of the splenium 

 curve around the posterior cornu and trigonum ventriculi of the lateral ventricle, 

 to form a thin but definite w r hite stratum, the tapetum, in the roof and ectal wall 

 of these parts of the cavity. 



The transverse direction of the fibres is rendered apparent in a dorsal view 

 of the exposed corpus callosum in the form of the striae transversae. These are 



