The cavity of the thalamencephalon persists as the _third ventricle_ or _diacoele_. The parts of the brain developed from the mid-brain and the hind-brain are much less complicated, and, except for variations in size, they present a fairly uniform character in most Fishes. In the mid-brain the roof bulges out into a pair of _optic lobes_, and by the growth of lateral thickenings in its floor two thick strands of longitudinally disposed nerve fibres, the _crura cerebri_, are formed. The cavity of the mid-brain remains as the _mesocoele_, and from it an extension may be prolonged into each optic lobe. From the hind-brain are formed the _cerebellum_ or _epencephalon_ and the _medulla oblongata_ or _metencephalon_, the former as a dorsal bulging, the latter as a ventral thickening. Except where the cerebellum is developed the dorsal wall remains epithelial, and forms the roof of the persistent cavity of the {371}hind-brain, the _fourth ventricle_ or _metacoele_, which retains its primitive continuity with the central canal of the spinal cord. Lateral lobe-like outgrowths from the dorsal columns of the medulla are conspicuous structures in some Fishes, and are known as _corpora restiformia_. The paired portions of the brain are connected across the middle line by a series of _transverse commissures_. The more important modifications of the brain in Cyclostomes and Fishes will now be briefly dealt with. [Illustration: FIG. 211.—Dorsal (A) and ventral (B) views of the brain of _Petromyzon marinus_. _ch.pl.1_, Anterior choroid plexus forming the roof of the prosencephalon and thalamencephalon; _ch.pl.2_, aperture in the roof of the mid-brain exposed by the removal of the middle choroid plexus; _ch.pl.3_, the fourth ventricle exposed by the removal of the posterior plexus; _cr.crb_, crura cerebri; _crb_, cerebellum; _crb.h_, cerebral hemispheres; _dien_, thalamencephalon; _inf_, infundibulum; _l.gn.hb_, left ganglion habenulae; _med.obl_, medulla oblongata; _nv.1_, olfactory; _nv.2_, optic; _nv.3_, oculomotor; _nv.5_, trigeminal; and _nv.8_, auditory nerves; _olf.l_, olfactory lobes; _opt.l_, optic lobes; _pn_, pineal organ; _r.gn.hb_, right ganglion habenulae. (From Parker and Haswell, after Ahlborn.)] {372}[Illustration: FIG. 212.—Dorsal view of the brain of _Myxine_. _c.r_, Corpora restiformia; _m.o_, medulla oblongata; _n.p_, naso-pituitary canal; _ol.o_, olfactory organ enclosed in its fenestrated cartilaginous capsule; _op.l_, optic lobes; _pr_, prosencephalon; _s_, _s_, dorsal roots of spinal nerves; _sp.c_, spinal cord; _th_, thalamencephalon. (From Wiedersheim, after Retzius.)]