102 



MESENCEPHALON. 



The tegmentum, like the formatio reticularis of the bulb and pons, of which 

 it is the prolongation upwards, is composed of small longitudinal bundles of white 

 fibres, separated by transversely coursing or arched fibres, together with a con- 

 siderable amount of grey matter containing scattered nerve-cells. 



In addition to these diffused bundles of longitudinal fibres there are others 

 which are collected into more defined tracts. One such tract constitutes the 

 posterior or dorsal longitudinal bundle, which is seen in all sections of this 

 part of the brain as a pyriform area of transversely cut fibres which lies on each side 

 of the middle line between the grey matter underlying the aqueduct, and the 

 formatio reticularis (fig. 76, p.l.l.}. The fibres which constitute this bundle below 



Fig. 76. TRANSVERSE SECTION ACROSS THE MID-BRAIN, THROUGH THE INFERIOR CORPORA QUA^RIGEMIN A. 

 MAGNIFIED ABOUT 3J DIAMETERS. (E. A. S.) From a Photograph. 



gr. , dorsal quadrigeminal groove (sulcus longitudinalis) ; c.q.p., corpus quadrigeminum postering; 

 str.L, stratum lemnisci ; c.gr. , central grey matter; .///, IV, oculo-motor nucleus; d. V, descending 

 root of fifth nerve ; p.l.b., posterior longitudinal bundle ; f.r.t., forrnatio reticularis tegmenti ; d, d', 

 decussating fibres of tegmentum ; s.c.p., decussating fibres of superior cerebellar peduncles ; /, upper 

 fillet; /', lower or lateral fillet ; p.p., pes pedunculi ; s.n., substantia nigra ; g.i.p., interpeduncular 

 grey matter ; Sy, Sylvian aqueduct. 



have already been noticed (see p. 65 and figs. 53, 54) ; traced upwards its fibres 

 become related to the nuclei of the third and fourth nerves, and mostly pass out with 

 the roots of these nerves. The posterior longitudinal bundle is composed exclusively 

 of large nerve-fibres, which acquire their myelin at an early stage, in fact, as soon 

 as the roots of the nerves themselves. Although its fibres are large, it gives off 

 principally fine nerve-fibres (Koppen). If this is the case they are probably 

 collaterals. It appears to be developed in nearly all vertebrates, and in some is 

 better marked than in mammals ; e.g., in the lizard it can be traced right down the 

 cord dorsal to the anterior or ventral commissure. Although mainly related to the 



