PONS VAROLII. 65 



of the pyramid, it there forms the pyramidal trad of the isthmus of Flechsig 

 (peduncular tract of Meynert). Some of the fibres of the pyramid, however, emerge 

 as external arcuate fibres, and joining the restiform body pass to the cerebellum. 

 Moreover, the pyramidal tract is larger in the pons and medulla oblongata than in 

 the spinal cord, for as it passes downwards it gives off numerous fibres to end by 

 terminal ramifications in the nerve-nuclei of the efferent cranial nerves. Collaterals 

 also pass off from the fibres of the pyramids, and even more numerously from the 

 longitudinal fibres of the posterior and lateral areas of the medulla oblongata and 

 pons to end by terminal ramifications in the grey matter (Kolliker). A part of the 

 lateral column of the cord forming the dorso-lateral cerebellar tract (see fig. 52, d.a.c.\ 

 passes at about the middle of the medulla oblongata obliquely backwards in the 

 restiform body to the cerebellar worm. The venire-lateral ascending cerebellar tract 

 (v.a.c.) passes into the upper part of the pons and turning dorsally at about the level 

 of the exit of the fifth nerve curves backwards and enters the cerebellum over and 

 along with the superior peduncle and in the valve of Vieussens. 



The rest of the lateral column runs up dorsal to the olives and contributes to 

 form the longitudinal fibres of the reticular formation. These are continued through 

 the dorsal parts of the encephalic isthmus towards the corpora quadrigemina and 

 optic thalamus. Some of the fibres become collected as they pass up towards the 

 pons into the well-marked flattened bundle of fibres (fillet) lying dorsal to the 

 pyramid. For the fillet is not, according to Edinger, wholly formed of the arched 

 fibres which emanate from the nuclei of the contra-lateral posterior columns ; it 

 receives an accession of fibres which have already crossed over in the spinal cord 

 from the posterior horn (through the anterior commissure), and have passed up the 

 antero-lateral column to the medulla oblongata. According to Flechsig and v. 

 Bechterew the fillet also contains fibres which are passing from the ventral auditory 

 nucleus through the trapezium to the inferior corpora quadrigemina. 



The anterior column of the cord in part is continued into the pyramid of the 

 same side, but chiefly dips under the pyramid and forms the longitudinal fibres of 

 the white reticular formation in the dorsal part of the mesial area. These pass 

 upwards towards the isthmus cerebri. In the pons Varolii one tract of them 

 becomes collected into a well marked fasciculus (posterior or dorsal lonyiludinal 

 bundle, figs. 53, 54, p. I), and some of the others join the fillet ; their further destina- 

 tion will be afterwards noticed. In the region of the medulla oblongata they are 

 indistinguishable from one another in the adult, but in the foetus they are found to 

 develope at different periods and are then readily differentiated (Flechsig). 



A small bundle of fibres of the anterior column of the cord was described by Solly as 

 passing: obliquely upwards below the olive, to join the restiform body (see fig. 30,/). This is 

 seldom distinct. 



Transition from the pons Varolii to the mid-brain. In sections through 

 the upper part of the pons (fig. 53) the fourth ventricle, which here becomes rapidly 

 narrowed, is roofed over by the two superior peduncles of the cerebellum which are 

 passing to the mesencephalon, and by the superior medullary velum and lingula, 

 which lies between them. The grey matter of the floor of the ventricle (central 

 grey matter) shows on either side near the median groove a group of nerve-cells con- 

 tinuous below with the nucleus of the funiculus teres, and above with the nucleus of 

 the aqueduct. More to the side is the mass of pigmented nerve-cells known as the 

 substantia ferrugiuea (fig. 53, s.f.), and still more laterally at the angle which the 

 roof makes with the floor, a column of large spherical scattered cells, along the 

 outer border of which a well marked white bundle the descending root of the fifth 

 nerve (v.d.*) runs downwards towards the middle of the pons where it issues with the 

 motor root. These cells and the fibres of the descending root can be traced upwards 



VOL. III. F 



