THE CEREBRAL HEMISPHERES 



955 



also present, and terminate in the molecular layer and presumably come via the anterior com- 

 missure from the mitral cells of the opposite bulb. 



The claustrum, although usually enumerated among the basal ganglia, is probably the thick- 

 ened and isolated deepest layer of fusiform cells belonging to the cortex of the island of Reil. 

 The white lamina 1 intervening between it and the cortex proper consists of association axones 

 of longer and shorter course 



Summary of the Cerebral Fibre Systems. The white substance of the cerebrum 

 consists of myelinic fibres intricately interwoven but permitting of classification 

 into three systems arranged according to the course they take. These systems 

 comprise: (1) association fibres, which connect neighboring or distant parts 

 within the same cerebral hemisphere; (2) commissural fibres, which unite allied 

 parts in the two cerebral halves and come transversely across the midline to form 

 the commissures; (3) projection fibres, which connect the cerebral cortex" with 

 lower centres in the brain and spinal cord, and, conversely, those fibres which 

 connect lower centres with the cerehraLcarlex. 



White substance (dorsal part) 



Neuroglia 



Glomerular layer 



Layer of olfactory nerve fibres 

 FIG. 708. Coronal section of olfactory bulb. (Schwalbe.^ 



1. The association fibres ( Fig. 709) connect different structures in the same hemi- 

 spheres, and are in or near to the cortex. They take origin from the small pyram- 

 idal and polymorphous cells of the deep layer of the cortex. Their direction is 

 parallel to the surface of the hemisphere, and in their course they cross the pro- 

 jection and eommissural fibres. They 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 cortex of 

 the hemispheres, and connect adjacent convolutions. They constitute stibcortical 

 tracts and are divided into arcuate fibres and tangential fibres. Thus, some of 

 these fibres connect the "visual sensor area with the visual memory area, and 

 the auditory sensor with the auditory memory area." 



The long association fibres associate cerebral centres which are far apart. They 

 are gathered into bundles and dip down deep into the centrum ovale. They 

 include the following: (a) the uncinate fasciculus; (6) the superior longitudinal 

 fasciculus; (c) the inferior longitudinal fasciculus (doubtful); (d} the cingulum; and 

 (e) the fasciculus rectus. 



1 Previously described as the periclaustral lamina or capsula eitrema (p. 949). 



