PROJECTION FIBRES. 163 



INTIMATE STRUCTURE OF THE CEREBRAL HEMISPHERES. 



STRUCTURE OF THE WHITE MATTER. 



The cerebral hemispheres, like the rest of the encephalon, are composed of white 

 and grey substance, the white pervading nearly the whole of the middle of each 

 hemisphere, where it forms what is known as the medullary centre, and extending 

 into the convolutions ; the grey forming a covering of some thickness over the 

 whole surface of the convolutions (cortex), and occurring also at the base of the 

 hemisphere in the form of the so-called basal ganglion (corpus striatum). 



The white matter consists of medullated fibres, varying in size in different parts, 

 but in general smaller than those of the cord and bulb. They are arranged in 

 flattened bundles separated by neuroglia ; the bundles have a somewhat rod-like 

 appearance in transverse section. 



The fibres of the medullary centre, though forming many different groups, may 

 be referred to three principal systems, according to the general course which they 

 take, viz. : 1. Projection-fibres, which pass from the isthmus encephali to the 

 hemispheres, or vice versa. These fibres increase in number in passing the optic 

 thalami and corpora striata, beyond which they spread in all directions into the 

 hemispheres. 2. Transverse or commissural fibres, which connect the two hemi- 

 spheres together. 3. Association-fibres (Meynert), which, keeping on the same side 

 of the middle line, connect near or distant parts of the same hemisphere. 



1. The projection fibres in each hemisphere are continuous in part with the 

 fibres of the crusfca, in part with those of the tegmentum, the latter probably 

 indirectly through the corpus striatum and optic thalamus. They are in great 

 measure, if not wholly, direct prolongations of the axis-cylinder processes of cells of 

 the cortex (see diagram, fig. 20, p. 23). 



a. The fibres which are continuous with those of the crusta pass in the internal 

 capsule, between the optic thalamus and nucleus caudatus mesially, and the nucleus 

 lenticularis laterally, probably giving off collateral fibres to those ganglia. Beyond 

 the internal capsule the fibres diverge into the general white matter of the hemi- 

 spheres, forming part of the system of radiating fibres known from its fan-like 

 arrangement as the corona radiata (Reil) or fibrous cone (Mayo), the latter tsrm 

 being derived from the way in which the assemblage of radiating fibres is curved 

 round in the form of an incomplete hollow cone as it emerges from below the 

 nucleus caudatus, which follows the curve of the lateral ventricle. 



Although it is probable that most of the fibres of the crusta pass directly into the 

 medullary centre and through this to the grey cortex, without entering the basal ganglia 

 of the hemispheres, this has only been definitely ascertained for one or two of the 

 tracts of fibres which run in the crusta. The best known of these is the pyramidal 

 tract, which is traceable through the inner capsule (opposite the middle of the lenti- 

 cular nucleus) and corona radiata to the grey cortex of the ascending frontal and 

 ascending parietal convolutions and to the posterior parts of the first and second 

 frontal gyri. This is of interest in connection with the fact that physiological 

 experiment indicates the grey matter of these particular convolutions as especially 

 concerned in governing the action of the chief groups of muscles of the body (kinaes- 

 thetic or psycho-motor centres). 



Another group of projection-fibres is the so-called direct sensory tract, which 

 passes from the external or lateral part of the crusta through the posterior part of 

 the internal capsule into the white matter of the occipital and temporal lobes of the 

 hemisphere (centres for special senses). 



The projection-fibres from the prefrontal region pass downwards in the anterior 

 part of the internal capsule. 



M 2 



