994 COMMISSURAL FIBRES. [BOOK m. 



becomes less conspicuous (Fig. 112, Z) though maintaining its 

 position dorsal to the reticular formation, and at the hind end 

 of the bulb appears to be a continuation forwards of those fibres, 

 "ground fibres," of the anterior column of the cord which probably 

 serve as successive short longitudinal commissures between the 

 segments of the cord. While the somewhat analogous fillet runs 

 ventral to the reticular formation, this posterior longitudinal bundle 

 runs always dorsal to that structure. It may be traced forward as 

 far as the nucleus of the third nerve, and is seen in transverse 

 sections lying immediately ventral to that group of cells (Fig. 

 114, I.), but its further connections forward have not as yet been 

 determined. It is relatively more prominent in the lower than 

 in the higher animals, and its fibres acquire their medulla 

 relatively early. It is supposed to be connected with the nuclei 

 of the nerves governing the muscles of the eye, and so to be 

 concerned in the movements of that organ. 



Tracts from the corpora quadrigemina. From each corpus 

 quadrigeminum there passes obliquely forwards and downwards on 

 each side a band of fibres, connected with the grey matter of the 

 corpus and known as its brachium. The anterior brachium (Fig. 

 114, Ba), as we shall see in dealing with the optic nerve, joins the 

 lateral corpus geniculatum and helps to form the optic tract, but 

 some of its deeper lying fibres proceed to the occipital cortex 

 forming part of the fibres which we have ( 633) described as 

 passing from the occipital cortex to and past the thalamus. The 

 posterior brachium passes to the median corpus geniculatum ; 

 having received fibres from, and probably given fibres up to that 

 body, it is continued on to the tegmentum, and according 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. 



635. 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 

 forms the corpus callosum. Starting from the cortex in nearly all 

 parts of the hemisphere, the fibres converge towards 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 supposed that by means 

 of these fibres, each part of the cortex of one hemisphere is 

 brought into connection with the corresponding part of the other 

 hemisphere. 



