THE OPTIC LUBl^S. 



151 



duct, hence in a transverse section taken in front of this point 

 (Fig-. 104 //') the cavity of either side appears to be isolated; in 

 a horizontal and longitudinal section (Fig-. T05 Aq) the general 

 arrangement of the parts may be well seen. 



The grey matter „. 



^ '' Fig. 104. 



is chiefly arranged 

 in a layer so as to 

 surround the cavity 

 (Fig-. 104), this layer 

 being deeper on 

 either side of the 

 descending portion 

 of the cavity than 

 elsewhere. For the 

 rest the grey matter 

 is much interspersed 

 among the white 

 matter, except at the 

 circumference of the 

 section, which is en- 

 tirely formed of white 

 matter. It contains 

 a large number of 

 small cellSj of which 

 the nuclei are alone 



visible ■ in the parts Transverse section through the anterior jjortion of the optic lobes 

 . ' opijosite the origin of the motor-oculi nerve ; from Stieda (mag- 



mentioned where the nified 30-80). 



I^Dwer portion of cavity. 



Lateral portion of cavity. 



GangUa of oculi- motor nerve. 



Oculo-motor nerve. 



Large cells of the optic lobe. 



Roof of optic lobe. 



Posterior commissure. 



Anterior diverticulum of the cavity. 



layer is most marked 

 these cells are ar- 

 ranged in oblique 

 rows, between which 

 pass flne bundles of 

 medullated fibres 

 (Fig. T04, between 71 and //). A group of large cells (Fig. 104 v) is 

 found on either side of the middle line and under the floor of the cavity ; 

 the cells are about 0-032 mm. long, and 0*0 16 mm. broad ; the oculo- 

 motor nerve may be traced to this group, which is the oculo-motor 

 nucleus. A small commissure of decussating fibres connects the 

 nuclei of opposite sides (Koppen). 



The substance of the roof of the optic lobes (Fig. 104 x) is 

 arranffed in verv distinct la vers : above is a laver free from cells ; 



