152 THE NERVOUS SYSTEM. 



the fibres of this layer are extremely fine and delicate, and have 

 not been accurately traced ; under this is a nuclear layer ; a second 

 layer of fine fibres follows, which is succeeded by a second layer of 

 nuclei in a granular matrix, and lastly epithelium (Stieda). (Reissner 

 describes three nuclear layers in Biifo variabilis, and this is also the 

 ease in R. temjooraria, G. H. ; according' to Koppen, the number is 

 variable.) In the anterior portion of the roof a distinct bundle of 

 fibres may be made out ; externally they bend downwards, and can 

 be traced as far as the crura cerebri. 



Fibres corresponding with the commissural and arched fibres of 

 the medulla oblongata are continued into the hinder portion of the 

 crura cerebri, the change from pars commissnralis to crura cerebri 

 being very gradual. The longitudinal white fibres are much in- 

 creased in number in the c7-ura cerebri, and a portion of them can 

 be traced to the nucleus magnus. 



A peculiar irregular group of large cells (Fig. 104 iv) is found 

 where the roof meets the crura cerebri ; these vary in diameter from 

 O"034 to 0*040 mm., and their processes are very indistinct. 



The fibres of the optic tracts arise, according to Koppen, from 

 two different origins : the one lies on the hindermost part of the 

 optic lobe ; from this point the fibres curve downwards and forwards 

 to form longitudinal fibres; this root Koppen names the ventral 

 ascending root, it can be traced through the entire length of 

 the organ. The second root arises in the tectum opticum near the 

 longitudinal fissure; it is smaller than the foregoing, and has 

 been named the dorsal ascending optic root. The fibres of these 

 two roots unite anteriorly near the posterior commissure, at which 

 point they receive additional fibres (Koppen). Bellonci traces a 

 large proportion of the fibres of the optic tract to the nucleus 

 magnus, which pair of nuclei, as already stated, he regards as the 

 posterior pair of corpora qioadrigemina of higher animals. 



The pars peduncularis is the continuation of the pars commissn- 

 ralis underneath the optic lobes ; a gelatinous mass lying in the 

 median plane and containing numerous isolated nuclei {Ganglio^i 

 interpedunculare) divides it into two lateral halves. The longitu- 

 dinal fibres are ungrouped posteriorly, but arranged in rounded 

 strands in the middle, especially dorsally; anteriorly the grouped 

 arrangement is lost and the number of fibres diminished. 



The pigment of this region has, in a transverse section, an outline 

 which has something the form of a lyre ; commencing on either side 

 of the median line, and underneath the deepest portion of the cavity. 



