98 
ZOOLOGICAL EXERCISES. 
left optic lobe goes to the right eye, and vice versa. 
The posterior portion of the brain is single. It is 
called the cerebellum. From it proceed a large 
number of nerves, of which the most important are 
the fifth parr passing forward, which divide into 
several branches going to the mouth and operculum, 
and the eighth pair passing backward and sending 
branches to the gills. A large branch passes directly 
backward, and forms the vagus nerve already seen 
when taking off the skin. Divide one of the optic 
lobes with a scalpel. It is hollow inside, and the 
cavity is called the cerebral ventricle. 
12. Remove the eye. Notice the six muscles attached to 
it—four rectt, and two oblique. Notice the transpa- 
rent corned, under which the golden yellow i7%s forms 
a ring round the opening, or pupil. The white outer 
coating of the eye-ball behind the iris, is the sclerotic. 
Fasten the eye to a loaded cork by pins through the 
muscles. Prick the cornea, the aqueous humour will 
escape. With the scissors cut all round the junction 
of the iris with the sclerotic, and remove it. The 
spherical, transparent crystalline lens will be seen in 
the centre. Remove it. Below will be seen the 
transparent gelatinous vitreous hwmour. Cover with 
water. Remove the vitreous humour with a brush. 
With a pair of forceps lift up the whitish retina (it 
is quite transparent in its original state) and notice 
below it the black choroid. Remove the choroid, 
and the sclerotic will be left. With a knife scrape 
some of the iris from the cornea. 
