66 



THE MEDULLA OBLONG ATA AND PONS VAKOLII. 



as far as the superior corpora .quadrigemina. Between the central grey matter and 

 the reticular formation on either side of the raphe is the now sharply defined dorsal 

 (posterior) longitudinal bundle (p.l.). The fibres which compose this, which receive 



Fig. 53. TRANSVERSE SECTION THROUGH THE UPPER PART OF THE PONS (Schwalbe, after Stilling). 

 Kather more than twice the natural size. * 



p, transverse fibres of the pons ; py, py, bundles of the pyramids ; a, boundary line between the 

 tegmental part of the pons and its ventral part ; I', oblique fibres of the lateral fillet, passing towards the 

 inferior corpora quadrigemina; I, lateral; I 2 , mesial fillet; f.r., formatio reticularis ; p.l., posterior 

 longitudinal bundle ; s.c.p., superior cerebellar peduncle ; v.m.a., superior medullary velum ; I, grey 

 matter of the lingula ; v 4, fourth ventricle ; in the grey matter which bounds it laterally are seen, 

 v.d., the descending root of the fifth nerve, with its nucleus ; s.f., substantia ferruginea ; g.c., group 

 of cells continuous with the nucleus of the aqueduct. 



their myelin sheaths very early, serve partly as a medium of communication between 

 the nucleus of the sixth nerve and the third and fourth nerves, and are partly pro- 

 longed downwards towards the spinal cord and upwards towards the optic thalamus. 



Fig. 54. SECTION ACROSS THE JUNCTION OF THE PONS 



VAROLII AND MID-BRAIN AT THE PLACE OF EXIT OF THE 



FOURTH NERVE (Stilling). THE IORSAL PART ONLY OF 



THE SECTION IS REPRESENTED. 



Aq, aqueduct of Sylvius ; IV, issuing fourth nerve ; 

 IF', its bundles decussating in the valve of Vieussens ; 

 IV", a bundle cut across in the central grey matter of the 

 aqueduct ; d. V, descending root of the fifth nerve ; 

 p.l, posterior (dorsal) longitudinal bundle; r, reticular 

 formation ; I, lateral fillet ; s.c.p, superior cerebellar 

 peduncle. 



The fillet is also now more distinct from the neighbouring longitudinal bundles of 

 the reticular formation, and a considerable part of it, known as the lateral fillet, is 

 seen to be passing to the side of the pons, where its fibres as they course obliquely 

 towards the corpora quadrigemina overlap the superior cerebellar peduncle of the 

 same side. As the fillet thus passes gradually to the side it gives place to a round 



1 The details of this and of several of the preceding figures are filled in under a somewhat higher 

 magnifying power than that used for tracing the outlines. 



