THE HEMISPHERES. 477 



from these bodies would reach almost immediately the convolutions of 

 the Island of Reil. 



Intimate Structure of the Hemispheres. As the longitudinal tracts 

 of white substance pass upward and forward from the medulla oblongata 

 and tuber annulare to the base of the hemispheres, they form the two 

 irregularly cylindrical masses known as the " crura cerebri." Their 

 fibres plunge, for the most part, directly into the substance of the cor- 

 pora striata in front, and of the optic thalami behind ; while from these 

 two pairs of ganglia other fibres emerge into the white substance of the 

 hemispheres. A portion of the fibres, however, coming from the crura 

 cerebri, pass, according to Vulpian, Henle, and Meynert, uninterruptedly 

 onward, in the internal capsule between the optic thalamus and corpus 

 striatum, and between the two nuclei of the corpus striatum; thus 

 reaching the white substance of the hemispheres without having tra- 

 versed the gray matter of the ganglia. 



From this level, that is, above and outside of the cerebral ganglia, the 

 ascending divergent tracts of white substance form a spreading crown 

 of nerve fibres, which radiate in all directions to the layer of gray sub- 

 stance on the exterior. These fibres are of very small size, as compared 

 with those in the peripheral portion of the nervous system ; their aver- 

 age diameter, according to Kolliker, being 4.5, and none of them being 

 larger than 6.7 micro-millimetres in thickness. 



The general direction of the radiating nerve fibres is easily seen, in 

 brains hardened in alcohol, to be that described above. At the same 

 time, microscopic examination shows that they are abundantly mingled 

 with other fibres which cross them at right angles, or nearly so, and 

 which are to be found more or less in every portion of the white sub- 

 stance of the hemispheres. These horizontal or curvilinear cross fibres 

 are derived in great measure from the lateral expansions of the corpus 

 callosum, the transverse fibres of which spread out in various direc- 

 tions, bending toward the convolutions of the upper, middle, and lower 

 part of the hemispheres. As these fibres come from the median line at 

 the level of the corpus callosum. they necessarily cross those which are 

 ascending from the cerebral ganglia below. The corpus callosum is 

 consequently a great transverse commissure, uniting the two hemi- 

 spheres of the cerebrum with each other. There are also fibres imme- 

 diately beneath the cortical layer of gray substance, following the curve 

 of the convolutions where they project inward into the white substance. 

 These are known as the " arciform fibres," and are regarded as connect- 

 ing filaments between the gray substance of adjacent convolutions. 



The gray substance of the cerebral convolutions forms an external 

 layer, from two to three millimetres in thickness, into which the fibres 

 of the white substance penetrate from within in a perpendicular manner. 

 It consists of a uniform granular matrix, in which are imbedded the 

 nerve cells, and through which the fibres continue their course in va- 

 rious directions. The nerve cells are rounded or irregular in form, with 

 a varying number of simple or branched prolongations. In the middle 





