678 THE NERVOUS SYSTEM. 



include : (a) the transverse fibres of the corpus callosum ; (>) the- anterior com- 

 missure ; (c) the posterior commissure, and have already been described. 



3. Association Fibres. These connect different structures in the same hemi- 

 sphere, and are of two kinds: (1) Those which unite adjacent convolutions, short 

 association fibres ; (2) those which pass between more distant parts in the same 

 hemisphere, long association fibres. 



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

 stance of the cortex of the hemispheres, and connect together adjacent con- 

 volutions. 



The long association fibres include the following : (a) the uncinate fasciculus ; 

 (6) the cingulum ; (c) the superior longitudinal fasciculus ; (d) the inferior longi- 

 tudinal fasciculus ; (e) the perpendicular fasciculus ; and (/) the fornix. 



(a) The uncinate fasciculus passes across the bottom of the Sylvian fissure, 

 and connects the convolutions of the frontal lobe with the anterior end of the 

 temporal lobe. 



(6) The cingulum is a band of white matter which encircles the hemisphere in 

 an antero-posterior direction, lying in the substance 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 passes into the hippocampus major, through 

 which it courses to its anterior extremity. 



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

 hemisphere, and connects the frontal and occipital and the frontal and temporal 

 lobes. 



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

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

 and posterior cornua of the lateral ventricle. 



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

 the occipital lobe, and connects the inferior parietal lobule with the fourth temporal 

 convolution. 



(/) 'V\\e fornix connects the hippocampal convolution with the corpus albicans, 

 and, by means of the bundle of Vicq d'Azyr, with the optic thalamus (see page 

 672). Through the fibres of the lyra it probably also unites the opposite hippo- 

 campal convolutions. 



The gray matter of the cerebrum is disposed in two great groups : (1) The gray 

 matter of the cerebral cortex. (2) The gray matter of the basal ganglia, the 

 nucleus caudatus and the nucleus lenticularis of the corpus striatum ; the claustrum 

 and the amygdaloid nucleus. They are, with the exception of the amygdaloid 

 nucleus, situated to the inner side of the island of Reil, and form with this convo- 

 lution the oldest part of the hemisphere, for they are the first parts of the enceph- 

 alon to be differentiated in the development of the individual. They are simply 

 semi-detached local thickenings of the gray cortex. The optic thalamus is not 

 reckoned as a basal ganglion, but as belonging to the thalamencephalon. 



GRAY MATTER OF THE CORTEX. 



On examining a section through one of the convolutions of the Rolandic area 

 w'th a lens, it is seen to consist of alternating white and gray layers thus disposed 

 from the surface inward : (1) a thin layer of white substance ; (2) a layer of gray 

 substance ; (3) a second layer of white substance (outer band of Baillarger or 

 band of Gennari) ; (4) a second gray layer ; (5) a third white layer (inner band of 

 Baillarger) ; (6) a third gray layer, which rests on the medullary substance of the 

 convolution. 



The cortex is made up of nerve-cells which vary in size and shape, and of nerve- 

 fibres, which are either medullated or naked axis-cylinders, embedded in a matrix 

 of neuroglia. 



