Iff* 



CHAP. IL] THE BRAIN. 



Besides these callosal fibres from one hemisphere to another, 

 the white matter of each hemisphere contains fibres called" "asso- 

 ciation 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. 120, A) and, in part of its 

 course, lies along the thalamus (Fig. 117, Ca) is really a com- 

 missure 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. 116, Ca), which we shall later on speak of as the temporo- 

 sphenoidal convolution and in which callosal fibres are deficient, 

 whence it arches forward through the globus pallidus, past the 

 thalamus (Figs. 123, ca, 117, Ca) to the front part of the third 

 ventricle. It may be remarked that this commissure is still found 

 in those lower animals which do not possess an obvious corpus 

 callosum. 



The small posterior commissure may be regarded as mainly 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. 11,5, c), though it contains transverse 

 fibres, is in the main a collection of grey matter, indeed a part of 

 the central grey 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 commissural 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. 120, /), and 

 apparently end in the grey matter of the corpora albicantia. 

 Whether the band of fibres, known as Vicq d'Azyr's bundle (Fig. 

 116, Vb), which running in the lateral 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 

 fornix 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 start chiefly from the 

 nucleus dentatus, or other internal grey matter, and to those of 

 the inferior peduncle which start chiefly from the superficial grey 

 matter of the vermis, appear to start from the superficial grey 

 matter of the whole surface, from that of the median vermis as 



