LONGITUDINAL FIBRES OF THE TEGMENTAL SYSTEM. 633 



valve of Vieussens (Fig. 135, Via); but, still converging, they soon sink 

 ventrally beneath the posterior corpora quadrigeniina and at the level of the 

 junction between the anterior and posterior corpora quadrigeniina meet and 

 decussate ventral to those bodies in the ventral region of the tegmentum 

 (Fig. 136, S. P.). Beyond the decussation they are continued forward in 

 the tegmentum ventral to the anterior corpora quadrigemina as two strands, 

 one on each side, which appear to end in the red nuclei. 



In this way the peduncles connect certain parts of the gray matter of the 

 cerebellum with the tegmental region, and more particularly with the red 

 nucleus, and thus indirectly with the structures with which that region is 

 itself connected. 



The fillet This, as we have seen ( 525), takes origin in the bulb, in the 

 inter-olivary layer between the inferior olives, from fibres which are derived 

 through the supra-pyramidal or sensory decussation from the gracile and 

 cuneate nuclei. From this origin it passes forward on each side as a flat 

 band into the tegmental region of the pons, receiving accessions from the 

 superior olive and other collections of gray matter, and dividing there into 

 two strands, the median (Figs. 135, 136, Fin) and lateral (Figs. 135, 136, Fl, 

 and Fig. 131, B F) fillet. The lateral division ends partly in the gray 

 matter of the posterior corpus quadrigeminum, and partly in the white 

 matter underlying (Fig. 137, dm) the anterior corpus quadrigeminum ; the 

 median division, passing further forward appears partly to end in the gray 

 matter of the anterior corpus quadrigeminum, but partly to be continued on 

 to the subthalarnic region of the tegrneutum ventral to the thalamus, thence 

 to the thalamus, and so to the cortex. 



The longitudinal posterior bundles. In a transverse section through the 

 fore part of the pons at the level of the posterior corpora quadrigemina a 

 rather conspicuous bundle of longitudinal fibres (called the longitudinal 

 posterior bundle) is seen on each side, cut transversely, in the dorsal region 

 of the tegmentum just ventral to the nucleus of the fourth nerve (Fig. 136, 1). 

 Traced backward from the aqueduct beneath the fourth ventricle, it becomes 

 less conspicuous (Fig. 135, /), though maintaining its position dorsal to the 

 reticular formation, and at the hind end of the bulb appears to be a con- 

 tinuation forward 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, as is seen in transverse sections lying imme- 

 diately ventral to that group of cells (Fig. 137, /), 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 forward and downward on each side a band of fibres, 

 connected with the gray matter of the corpus and known as the brachium. 

 The anterior brachium (Fig. 137, 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 form- 

 ing part of the fibres which we have ( 545) 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, ac- 



