236 PROFESSOR F. M. BALFOUR. 



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

 optic nerve and several pairs of sensory nerves already described 

 (fig. 19 D, e) Avhile 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. 234), there is a more 

 or less distinctly defined lobe of white matter on the dorsal 

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

 matter. It is shown in the transverse sections (fig. 19 f and 

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

 mainly characterised by the presence on the ventral border of 

 definite transverse commissural fibres. 



The Skin. 



The skin is formed of three layers. 



1. The cuticle. 



2. The epidermis or hypodermis. 



3. The dermis. 



The cuticle is a layer of about 0"002 mm. in thickness. Its 

 surface is not, however, smooth, but is everywhere, with the 

 exception of the perioral region, raised into minute secondary 

 papillae, the base of which varies somewhat in diameter, but is 

 usually not far from 0*02 mm. On the ventral surface of the 

 body these papillae are for the most part somewhat blunt, but 

 on the dorsal surface they are more or less sharply pointed. In 

 most instances they bear at their free extremity a somewhat 

 prominent spine. The whole surface of each of the secondary 

 papillae just described is in its turn covered by numerous 

 minute spinous tubercles. In the perioral region, where the 

 cuticle is smooth, it is obviously formed of two layers which 

 easily separate from each other, and there is I believe a similar 



