634 THE BRAIN. 



cording to some authors, through the tegmentum by the hind part of the 

 hind limb of the internal capsule to the temporal region of the cortex, 

 mingling in its course with fibres from the thalamus. 



TRANSVERSE OR SO-CALLED COMMISSURAL FIBRES. 



548. The two chief masses are those, on the one hand, belonging to 

 the cerebrum, and those, on the other hand, belonging to the cerebellum. 



In the cerebrum the most imposing mass of transverse fibres form the 

 corpus callosum. Starting from the cortex in nearly all parts of the hem- 

 isphere, the fibres converge toward the thick body of the corpus callosum 

 placed in the middle line, and thence diverge to nearly all parts of the 

 cortex of the hemisphere on the other side, interlacing in their course 

 with the cortical fibres of the pedal and tegmental systems. It is sup- 

 posed that by means of these fibres, each part of the cortex of one hemi- 

 sphere is brought into connection with the corresponding part of the other 

 hemisphere. 



Besides these callosal fibres from one hemisphere to another, the white 

 matter of each hemisphere contains fibres called " association fibres," passing 

 from one convolution to another of the same hemisphere. 



The small anterior white commissure though it is placed in the front part 

 of the third ventricle (Fig. 143, A) and, in part of its course, lies along the 

 thalamus (Fig. 140, Ca), is really a commissure of particular parts of the 

 cerebral hemispheres. A portion, very small in man, belongs to the olfactory 

 tract ; the rest takes origin on each side in a limited portion of the cortex 

 (Fig. 139, Ca\ which we shall later on speak of as the temporo-sphenoidal 

 convolution, and in which callosal fibres are deficient, whence it arches for- 

 ward through the globus pallidus, past the thalamus (Figs. 146, ca, 140, Ca) 

 to the front part of the third ventricle. It may be remarked that this com- 

 missure is still found in those lower animals which do not possess an obvious 

 corpus callosum. 



The small posterior commissure may be regarded as simply a commissure 

 between the two thalami, but it also helps to unite the tegmentum of the two 

 sides, and some fibres are said to pass on each side into the hemisphere. The 

 middle or soft commissure of the third ventricle (Fig. 138, c), though it con- 

 tains transverse fibres, is, in the main, a collection of gray matter, indeed, a 

 part of the central gray matter. 



The fornix, together with, at all events, part of the septum lucidum 

 which joins it with the corpus callosum, must also be regarded as a com- 

 missural structure. But its relations are peculiar ; for while, behind, the 

 diverging posterior pillars begin in the cerebral hemispheres, namely, in the 

 walls of the descending horn of the lateral ventricle on each side, in front 

 the anterior pillars or columns, leaving the cerebral hemispheres, pass along 

 the lateral walls of the third ventricle (Fig. 143, /), and apparently end 

 in the gray matter of the corpora albicantia. Whether the band of fibres, 

 known as Vicq d'Azyr's bundle (Fig. 139, Vb~), which running in the lat- 

 eral wall of the third ventricle leads dorsally from each corpus albicans up 

 to the anterior nucleus of the thalamus, is really to be considered as a 

 continuation of the foruix is disputed ; it may more probably be regarded 

 as a part of the system spoken of above as connecting the cortex with the 

 thalamus. 



In the cerebellum true commissural fibres are supplied by the middle 

 peduncles; but by no means all the fibres of these peduncles are of this 

 nature. The fibres of the middle peduncle, in contrast to those of the 

 superior peduncle which starts chiefly from the nucleus dentatus, or other 



