VERTEBRATA. 407 
fourth ventricle. The part in the optic lobes becomes con- 
stricted into a small canal or z#er leading from third to 
fourth ventricles. 
Fig. 293.—DIAGRAM OF THE 
VERTEBRATE BRAIN. 
(Mainly after Hux.ey.) 
Cerebrum. Pineal Body. 
Optic 
Lobe. Cerebellum. Spinal Cord. 
Crura 
Cerebri. 
The dorsal wall of the thala- 
mencephalon is produced into a 
process called the prneal body, 
which, in some cases, shows 
evidence of being a vestigial eye. 
The ventral wall is also produced 
into a process called the infundt- 
bulum, coming into relation with 
the pituitary body (v.i.); the 
lateral walls become thickened 
and form the optic thalami. 
Thus the brain becomes a com- 
plex organ consisting of a linear 
series of specialised portions ; 
but a further complication takes 
-place in the flexure of one part 
upon another. In the highest 
types (mammals) the brain is 
twice flexed upon itself and its 
+ et 
Fourth \ 
Notochord. 
Ventricle. 
Fig. 294. — DIAGRAM- 
MATIC MEDIAN SECTION 
THROUGH A VERTEBRATE 
BRAIN, SHOWING THE 
VENTRICLES, 
Lateral _ 
Ventricle. 
Third Ventricle. 
Iter. 
Fourth Ventricle. 
origin from a single tube is thus disguised. . 
From the brain there arise at least ten pairs of cranial 
nerves which are remarkably constant in their relationship. 
The fore-brain gives rise to the olfactory (I.) and optic 
(II.), the mid-brain to the oculomotor (III.) and trochlear 
(IV.), and the hind-brain to the trigeminal (V.), abducens 
