TRACT OF THE FILLET. 103 



sixth, fourth and third nerve-roots, fibres also pass from it to the auditory nucleus, 

 and others to the cerebellum. After giving off fibres to the root of the third nerve the 

 posterior longitudinal bundle is continued into the posterior commissure (see p. 109), 

 and partly upwards into the subthalamic region (? to the substantia interansalis, see 

 p. 112). The posterior longitudinal bundles come close together at the raphe, and 

 fibres pass from one to the other. These are probably the fibres which effect a crossed 

 connection between the abducens nucleus of the one side and the fibres passing to 

 the internal rectus by the third nerve of the other side (see pp. 63 and 99). Perhaps, 

 also, a connection is established through the posterior longitudinal bundle between 

 the facial and the oculomotor nucleus (Mendel). The posterior longitudinal bundle 

 is very small in the mole (Forel), large in reptiles and amphibia (Spitzka). 



Brachium conjunctivum ; superior cerebellar peduncle. Another tract 

 of longitudinal and decussating fibres is derived from the superior peduncle of 

 the cerebellum, which we have already traced as it passes forwards over the 

 superior end of the fourth ventricle. Reaching the sides of the aqueduct as a 

 well-marked bundle, of semilunar shape in section (fig. 72, s.c.p), it gradually takes 

 a more ventral position as it is traced upwards in the mesencephalon, and its 

 fibres soon begin to pass across the raphe, decussating with those of the other side 

 (fig. 75 A, and fig. 76, s.c.p.}, the decussation extending as far upwards as the superior 

 pair of corpora quadrigemina. Having thus crossed to the opposite side the tract 

 in question pursues its course longitudinally upwards, appearing at first as a white 

 bundle, but higher up enclosing in its passage a tract of grey matter with numerous 

 large piginented cells, known as the nucleus of the tegmentum or red nucleus (fig. 

 75, B, and fig. 78, r.n.), and probably receiving an accession of fibres from these cells. 

 Above, the tract passes into the ventral part of the optic thalamus. Some fibres 

 do not cross, but enter the red nucleus of the same side. 



Between the two red nuclei a small white bundle (Meynerfs bundle) passes 

 backwards on either side of the raphe from the ganglion of the habenula near the 

 roof of the third ventricle to a small mass of grey matter which lies between the 

 crura (ganglion interpedunculare, fig. 76, g.i-p.). 



Tract of the fillet. The fillet, which, in sections across the upper part of the 

 pons, forms a considerable flattened bundle of longitudinal fibres at the ventral 

 border of the formatio reticularis, is traceable upwards into the ventral part of the 

 tegmentum. Soon, however, the large laterally situated part of this tract is seen to 

 pass obliquely outwards and emerge at the side of the crus cerebri, curving obliquely 

 over the outer side of the prolongation of the cerebellar peduncle (fig. 75, A, /), 

 and tending for the most part towards the inferior corpora quadrigemina. It is seen 

 on the surface as a band of obliquely curved fibres, occupying a triangular area at 

 the side of the tegmentum (fig. 76,/'), and it was to this band that the name of 

 fillet was originally applied by Reil. It is now known as the lower or lateral fillet. 

 It is reinforced by fibres from the superior medullary velum which also curve round 

 the superior cerebellar peduncle, and which are probably derived from the antero- 

 lateral ascending tract of the cord, for they undergo degeneration after section of 

 the cord (see below). The fillet is covered externally by a thin layer of grey matter 

 containing nerve-cells. 



But all the fibres of the tract of the fillet do not take the course above indicated. 

 Those nearest the middle line (mesial fillet) separate themselves from the rest, and 

 pass at the lower part of the mesencephalon into the crusta (see p. 101), where they 

 form a mesial bundle (Wernicke), which is traceable up into the subthalamic region, 

 where it joins the ansa lenticularis. Those next in order (middle portion) are for 

 the most part, according to Forel, continued upwards in the formatio reticularis 

 of the tegmentum, but many of the fibres become lost amongst its cells, and 

 are not traceable further as a distinct tract. According to Edinger, they have a cell- 



