140 JULIA WORTHINGTON. 
cylindrical flask, with its closed base at the anterior end, and 
with its open mouth in the form of a tube passing into the 
sheath of the spinal cord behind. At the cephalic face, in 
the centre of the area perforated by the olfactory fibres, the 
wall is °192 mm. thick. In the median dorsal line it is -168 
mm. thick over the fore part of the olfactory lobes, and 
increases gradually in thickness as it proceeds caudad, mea- 
suring 296 mm. over the habenular ganglia, and ‘6 mm. 
over the cerebellum and fore part of the medulla. It then 
decreases gradually until, passing into the capsule of the 
spinal cord, it is only 364 mm. thick. On the ventral 
surface the thickness is distributed a little differently. The 
cranium is ‘167 mm. thick under the anterior end of the 
olfactory lobes, and +224 mm. under the cephalic border of 
the cerebrum. As it passes under the ’tween brain, however, 
it becomes thinner, being only ‘168 mm. thick immediately 
anterior to the hypophysis. Caudad to the infundibular 
process it becomes thicker again, measuring here ‘28 mm., 
and under the central part cf the medulla it measured 
"296 mm. ‘lhe capsule becomes very much thicker as it 
approaches the notochord, and finally receives that structure 
within itself, surrounding it with a thin sheath, cf. Ayers 
and Jackson, 1900. ‘The lateral walls of the cranium 
measure "168 mm. in thickness by the side of the olfactory 
lobes, and then thin out to only *085 mm. beside the fore 
brain. At the side of the mid brain, just anterior to the 
foramen of exit of the trigeminus nerve, it is ‘207 mm. 
thick, and just cephalad of the ear it measures °267 mm. At 
the side of the spinal cord the thickness is *28 mm. 
The density of the cranial structure varies inversely with 
the thickness. Where the wall is thinnest the fibres are 
straight and very closely packed. As it increases in thick- 
ness they separate more and more, some of them becoming 
wavy ; and where it is thickest, as in the dorsal surface over 
the cerebellum and medulla, and just in front of the noto- 
chord, the fibres are comparatively loosely packed, and 
almost all of them are more or less wavy. At the hind 
