892 ANATOMY AND DEVELOPMENT 



necting the two halves of the ganglion. Laterally and some- 

 what ventrally it is prolonged into a horn (fig. 19 D, E, b], which 

 I propose calling the ventro-lateral horn. In front it is placed 

 in a distinct protuberance of the brain, which is placed ventrally 

 to and nearly in the same vertical plane as the optic nerve. 

 This protuberance is best shewn in the view of the brain from 

 below given in PI. 51, fig. 22. This part of the horn is charac- 

 terized by the presence of large vertically-directed bundles of 

 nerve-fibres, shewn in transverse section in fig. 190. Posteriorly 

 the diameter of this horn is larger than in front (fig. 19, E, F, G), 

 but does not give rise to a protuberance on the surface of the 

 brain owing to the smaller development of the median lobe 

 behind. 



The median lobe of the brain is also prolonged into a dorso- 

 lateral lobe (fig. 19, a), which, as already mentioned, is freely 

 exposed on the surface. On its ventral border there springs the 

 optic nerve, and several pairs of sensory nerves already de- 

 scribed (fig. 19 D, E), while from its dorsal border a pair of 

 sensory nerves also spring, nearly in the same vertical plane as 

 the optic nerves. 



Posteriorly where the dorsal surface of the brain is not 

 covered in with ganglion cells the dorso-lateral horn and median 

 lobe of the brain become indistinguishable. 



In the front part of the brain the median lobe of white matter 

 extends dorsalwards to the dorsal strip of ganglion cells, but 

 behind the region of the transverse prolongation of these cells, 

 into the white matter already described (p. 890), there is a more 

 or less distinctly defined lobe of white matter on the dorsal 

 surface, which I propose calling the postero-dorsal lobe of white 

 matter. It is shewn in the transverse sections (fig. 19 F and 

 G, c). It gradually thins away and disappears behind. It is 

 mainly characterized by the presence on the ventral border of 

 definite transverse commissural fibres. 



