THE CENTRAL NERVOUS SYSTEM. 75 
Similarly cut through and remove the neural arches of the 
vertebrae one by one from before backwards. 
Haamine and draw the central nervous system in situ, showing 
I. The brain. 
a. The dorsal surface of the brain: xote from before 
backwards the following parts, removing the pigmented 
membrane—pia mater—covering the several parts as 
you come to them. | 
i, The olfactory lobes: united together in the median 
plane by a commissure: they give off the 
olfactory nerves from their anterior ends, and are 
separated behind by slight constrictions from the 
hemispheres. 
ii. The cerebral hemispheres: smooth ovoid bodies 
which touch one another in the median plane 
but are not fused together. 
iii, The thalamencephalon: lozenge-shaped: _ lies 
immediately behind the hemispheres and between 
their diverging posterior ends: it is covered by a 
thick vascular membrane—the choroid plexus— 
through which passes the stalk of the pineal 
gland, a small body adherent to and removed 
with the roof of the skull. On removing the 
choroid plexus a small hole is seen in the roof 
of the thalamencephalon, connecting the hollow 
stalk of the pineal gland with the third ventricle. 
The thickened sides of the thalamencephalon are 
the optic thalami. 
iv. The optic lobes: two prominent ovoid bodies 
touching one another in the median line; forming 
the widest part of the brain. 
v. The cerebellum: a narrow transverse band im- 
mediately behind the optic lobes. 
vi. The medulla oblongata: the part of the brain 
behind the cerebellum : it is widest in front and 
gradually tapers towards its posterior end, where 
it is continuous with the spinal cord. It is 
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