410 CEREBRUM. 



second white, the third gray, the fourth white, the fifth 

 gray, and the sixth white. The outermost or cortical layer, 

 which to the eye is cineritious, is thus seen to he, under the 

 glass, white, and this seems also to accord with the obser- 

 vations of Mr. Grainger, thus reversing the supposed order 

 of the several layers. 



In the posterior convolutions the distinction between 

 some of these layers has been frequently seen with the 

 naked eye. The tubular fibres from the hemispheres pass 

 through these layers, and in relation to the convolutions 

 are, according to Mr. Solly, arranged in four different ways, 

 "first, some of them commencing from the convolutions of 

 the anterior, middle, and posterior lobes, pass through the 

 corpora striata, and forming the inferior layer of the crus 

 cerebri, pass through the pons Yarolii, so as to form the an- 

 terior columns of the spinal cord, the motor tract ; second, 

 others commencing in the nerves of sensation, after pass- 

 ing through the pons Varolii, and emerging from the 

 substance of the thalamus, terminate in the same convolu- 

 tions, constituting the sensory tract ; third, others passing 

 from one side of the brain to the other, and in apposition to 

 the internal surface of all the convolutions, are those fibres 

 which, collected into a mass, form between the hemispheres 

 that wide bridge, the great transverse commissure or corpus 

 callosum, to be presently described ; fourth, in contact with 

 all the convolutions are the fibres of the superior and in- 

 ferior longitudinal commissures, which connect together 

 those convolutions which are situated on the same side of 

 the mesial line or different portions of the same hemi- 

 spherical ganglion." 



Those fibres going from the anterior and posterior col- 

 umns of the cord, as from a common centre and spreading 

 out upon the convolutions, are called by Gall and Spurzheim 

 the diverging fibres. While those proceeding from the con- 

 volutions towards the centre of the brain are named con- 

 verging fibres. 



The convolutions are distinguished into primary and 

 secondary. The primary are those which are found to be 



