786 THE NERVOUS SYSTEM. 



Avhich help make up the projection fibres do so, not directly, but by the interpo- 

 sition of the optic thalarni and corpora striata, between which there are also con- 

 necting fibres which run through the internal capsule. 



2. The transverse or commissural fibres connect together the two hemispheres. 

 They include (a) the transverse fibres of the corpus callosum, and (6) the anterior 

 commissure. 



The corpus callosum, which has already been described, connects together the 

 two hemispheres of the brain, forming their great transverse commissure, pene- 

 trating into the medullary substance of the convolutions and intersecting the 

 fibres of the corona radiata. The fibres of the corpus callosum can each be traced 

 either as a direct prolongation of an axis-cylinder process of a pyramidal cell in 

 the gray matter of the cortex, or as a collateral from one of the projection-fibres 

 just described. 



The anterior commissure is a round bundle of white fibres Avhich is placed in 

 front of the anterior pillars of the fornix, and appears to connect the corpora stri- 

 ata. It passes outward through the lenticular nucleus of the corpus striatum on 

 each side, and then curves, somewhat twisted on itself, downward and backward 

 into the substance of the temporal lobe, where its fibres radiate from each other. 



3. Association-fibres connecting Different Structures in the Same Hemisphere. 

 These fibres are of two kinds : (1) those which connect adjacent convolutions, and 

 which are termed short association-fibres ; (2) those which connect more distant 

 parts in the same hemisphere the long association-fibres. 



The short association-fibres are situated immediately beneath the gray substance 

 of the cortex of the hemispheres, and connect together adjacent convolutions, 

 arching beneath the cortical matter which lies at the bottom of the fissures. 



The long association-fibres include the following : 



(a) The uncinate fasciculus connects the convolutions of the frontal and tem- 

 poral lobe. It passes across the bottom of the Sylvian fissure and traverses the 

 claustrum. 



(6) The fillet of the gyrus fornicatus or cingulum is a band of white matter 

 which encircles the hemisphere in an antero-posterior direction, lying in the sub- 

 stance of the convolution of the corpus callosum. Commencing in front at the 

 anterior perforated space, it passes forward and upward parallel with the rostrum, 

 winds round the genu, runs in the convolution from before backward immediately 

 above the corpus callosum, turns round its posterior extremity, and is continued 

 downward and forward in the hippocampal gyrus to its extremity. In its course 

 it is connected with the secondary convolutions of the gyrus fornicatus by short 

 arcuate fibres. 



(c) The superior longitudinal fasciculus runs along the convex surface of the 

 hemisphere and connects the frontal lobe with the temporal and occipital. 



(d) The inferior longitudinal fasciculus is a collection of fibres which connects 

 the temporal and occipital lobes, running along the outer wall of the middle and 

 posterior cornu. 



(e) The perpendicular fasciculus passes vertically through the front part of the 

 occipital lobe, and connects the inferior parietal convolution above with the pos- 

 terior part (fusiform lobule) of the fourth temporal convolution below. 



(/) The fornix connects the corpus albicans with the crochet or uncus of the 

 hippocampal convolution in the manner which has already been described. 



THE GRAY MATTER OF THE HEMISPHERE is disposed in two regions : 1. The 

 gray matter of the cerebral cortex ; 2. The gray matter of the basal ganglia ; 

 that is, the corpus striatum and nucleus amygdalae. As the last two have already 

 been described, there remains only the cortex to be considered. 



The gray matter of the cortex (Fig. 474) invests the surface of the hemi- 

 spheres, covering in the convolutions or gyri and lining the intervening fissures 

 or sulci. When a vertical section is made through a gyrus, it is found to be 

 made up of a white centre invested by a portion of the cortex, which last, if 

 examined microscopically, is found to consist of five separate layers, but to this 



