THE BRAIN 487 
The second primary vesicle of the brain forms the third 
region, that of the optic lobes (mesencephalon or mid-brain) 
in the adult brain. The floor and lateral walls form the 
thickened crura cerebri; the roof becomes the two optic lobes, 
which are hollow in almost all Vertebrates. In Mammals 
a transverse furrow divides each optic lobe into two (corpora 
quadrigemina). The cavity of the vesicle becomes much 
contracted, and forms the narrow iter or aqueduct of Sylvius, 
a canal connecting the third ventricle with the fourth. 
The third primary vesicle gives rise to the metencephalon, 
or hind-brain, or region of the cerebellum, and to the 
myelencephalon, or after-brain, or region of the medulla 
oblongata. 
In the metencephalon the roof develops greatly, and 
gives rise to the cerebellum, which often has lateral lobes, 
and overlaps the next region. In the higher forms the 
floor forms a strong band of transverse fibres—the pons 
Varolii. 
From the region of the medulla oblongata most of the 
cranial nerves are given off. Here the roof, partly over- 
lapped by the cerebellum, degenerates, becoming thin and 
epithelial, the cavity—called the fourth ventricle—is con- 
tinuous with the canal of the spinal cord. 
Summary 
(1) Cerebral hemispheres, prosencephalon, or 
fore-brain. Note commissures, olfactory 
lobes and nerves, and first and second 
First Embryonic | ventricles. 
Vesicle. (2) Optic thalami, thalamencephalon, or tween- 
brain. Note—(a) optic, (4) pineal, (c) 
pituitary outgrowths, and the third ven- 
tricle. 
(3) Optic lobes, mesencephalon, or mid-brain. 
Note crura cerebri, and the aqueduct of 
Sylvius. 
Median Embryonic | 
| (4) Cerebellum,-metencephalon, or hind-brain. 
Vesicle. 
Note pons Varolii. 
(5) Medulla oblongata, myelencephalon, or 
after-brain. Note rudimentary roof, 
fourth ventricle, and origin of most of 
the cranial nerves. 
Third Embryonic 
Vesicle. 
