164 THE CEREBRAL HEMISPHERES. 



A few of the fibres of the crusta (those nearest the inner or mesial side) do not 

 pass into the inner capsule and corona radiata, but are collected into the bundle 

 known as ansa lenticularis and pass outwards underneath the thalamus into the 

 nucleus lenticularis (see p. 112). 



b. The fibres which pass to the cerebrum in the tegmentum, are originally 

 constituted by the longitudinal bundles of the formatio reticularis of the medulla 

 oblongata. They are reinforced as they pass upwards by sets of fibres derived from 

 the superior peduncle of the cerebellum, and perhaps the middle peduncle ; from the 

 fillet ; from the deeper parts of the corpora quadrigemina, and from the nerve- and 

 other nuclei in the parts which they traverse. They become lost for the most part 

 in the subthalamic tegmental region and in the thalamus, but on the other hand, 

 from the outer side of the thalamus fibres stream outwards (see p. Ill), and joining 

 the general system of the corona radiata, diverge to nearly every part of the hemi- 

 sphere. Other fibres, apparently continuous with this same system, pass from the 

 posterior part of the thalamus into the optic tract. 



From the lower part of the thalamus anteriorly fibres emerge forming the bundle 

 known as the inferior peduncle of the thalamus, and curving round below the nucleus 

 lenticularis, pass into the white substance of the external capsule. 



2. The transverse or comxnissural fibres which connect the hemispheres 

 together include a. The transverse fibres of the corpus callosum. b. The fibres of the 

 anterior commissure. 



The fibres of the corpus callosum are derived from the cells of the grey cortex, 

 being either the direct prolongations of their axis-cylinder processes or collaterals 

 passing off from the projection fibres before mentioned (fig. 20, call.). When, there- 

 fore, a portion of cortex is removed or destroyed, certain fibres in the corpus callosum 

 undergo degeneration. By this means it may be determined that the anterior portions 

 of the callosum contain chiefly fibres derived from the frontal lobes, the posterior 

 chiefly fibres from the occipital lobes, and the middle portion from the intermediate 

 parts of the mantle. The fibres from a limited part of the cortex are not, however, 

 entirely limited to one part of the callosum, but show a tendency to scatter, so that 

 not only similar but also dissimilar parts of the two hemispheres are connected 

 through this commissure (Sherrington). A certain number of projection fibres also 

 pass across the callosum to the other hemisphere, and then turn downwards in the 

 internal capsule (Hamilton). Hence, after removal of certain parts of the cortex on 

 one side of the brain, some degenerated fibres are found in the pyramidal tract which 

 is mainly connected with the other side (see p. 31). 



The anterior commissure (fig. 113) is composed of a bundle of transverse fibres, 

 which chiefly connects the temporal lobes of the two hemispheres. The bundle is most 

 compact in the middle line, which it crosses at the front of the third ventricle just 

 in front of the pillars of the fornix : in a median section of the brain it presents an 

 oval section of 5 mm. long diameter, with its long axis from above down. From 

 this point it passes laterally as a twisted bundle of fibres curving backwards and 

 somewhat downwards through the ventral part of the globus pallidus, and below the 

 putamen of the lenticular nucleus. Its fibres then diverge in a fan-like manner 

 into the temporal lobe ; whether any pass by means of the external capsule to the 

 insula has not been satisfactorily determined. 



The fibres of the anterior commissure which pass into the temporal lobe form by 

 far the greater part of the commissure in man, and constitute what has been termed 

 by Granser the pars temporalis. Besides these fibres, there are others which are 

 derived from the lobus olfactorius (see p. 160), and which appear to connect the 

 olfactory tract of one side with the hippocampal gyrus of the opposite side. These 

 form the pars olfadoria of Ganser : this part is very slightly developed in man. 

 The anterior commissure forms the segment of a circle, with the convexity 



