886 



THE NERVOUS SYSTEM 



seen at this level, the white substance of each hemisphere is known as the centrum 

 semiovale. Horizontal sections passing below the body of the corpus callosum 

 involve the corpus striatum and thalamus, and the appearance of the white sub- 

 stance is modified accordingly (fig. 694). 



In the white substance of the cerebral hemispheres as a whole three main sys- 

 tems of fibres are recognised: — projection fibres, commissural fibres, and associa- 

 tion fibres. The projection fibres are those of a more or less vertical course, which 

 pass to and from the cortex of the hemisphere, associating it with the structures 

 below the confines of the hemisphere. The commissural fibres are those of a 

 transverse or horizontal course, which cross the mid-line and functionally connect 

 the two hemispheres with each other. The association fibres are those which 

 neither cross the mid-line nor pass beyond the bounds of the hemisphere in which 

 they arise, but instead associate the different parts of the same hemisphere — lobes 

 with lobes and gyri with gyri. The fibres which associate the cortex with the 



Fig. 698. — Coronal Section of Brain Passing Through the Pulvinar of the Thalamus 

 AND the Uncus of the Hippocampal Gyrus. (After Toldt.) 



Chorioid tela of — ^ Internal 



third ventricle \ ■ \ „ MnrtBBaBli,.;/,. ! '//!,////„'///// -trnwurw/M,.,.. . y cerebral vein 



Tail of caudate 

 nucleus 



Internal capsule »^:;=^ 



Chorioid plexus 

 of third ven- 

 tricle 



Habenular 

 commissure 



Posterior 

 commissure 



Opening of 

 aqueduct of 

 cerebrum 



Habenular 

 nucleus 

 Tail of caudate 

 nucleus 



Peduncle of 

 cerebrum 

 Post, recess of 

 interpeduncular 

 fossa 



Substantia 

 nigra 



Pons (Varoli) 



nuclei of the corpus striatum must also be classed as association fibres, since these 

 masses of grey substance are a part of the telencephalon, while by definition those 

 which associate the thalamus and hypothalamus with the cortex belong to the 

 projection system. Some of the fibre bundles of the above systems have already 

 been described in connection with the parts with which they are concerned. 



The projection fibres of the hemisphere comprise both ascending and descend- 

 ing fibres between the cerebral cortex and structures below the bounds of the hemi- 

 sphere, i.e., some arise in the structures below and terminate in the cortex; others 

 arise from the cortical cells and terminate in the structures below, including the 

 grey substance of the thalamencephalon, mesencephalon, rhombencephalon, and 

 spinal cord. The projection fibres are given different names in the hemisphere 

 according to their arrangement and the appearances to which they contribute 

 in the dissections. Beginning with the pyramidal fasciculi and the basis of the 

 peduncle, they contribute — (1) to the internal capsule and some to the external 

 capsule and (2) to the corona radiata. 



The internal capsule [capsula interna] is a band of white substance, consisting 

 of t ho ascending fibres from the nuclei of the thalamus, hypothalamus, and corpus 

 striatum, reinforced by the descending fibres from the cortex to these nuclei and 

 by those descending in the cerebral peduncle to terminate in the mesencephalon, 

 rhombencephalon and spinal cord. It is a broad, fan-like mass of fibres, which 

 increases in width from the base of the hemisphere upward, and which is spread 

 between the lenticular nucleus on its lateral aspect and the caudate nucleus and 



