XIV NERVOUS SYSTEM . 271 
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 
Fic. 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 thalam- 
encephalon ; 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 ; er.crb, crura cerebri ; cvb, cerebellum ; crb.h, cerebral hemi- 
spheres; dien, thalamencephalon ; inf. infundibulum; l.gn.hb, lett ganglion 
habenulae ; med.ob/, medulla oblongata ; nv.1, olfactory ; nv.2, optic ; nv.3, oculo- 
motor; 7v,5, trigeminal ; and nv.8, auditory nerves ; o/f./, olfactory lobes ; opt./, 
optic lobes ; pn, pineal organ; 7.gn.hb, right ganglion habenulae. (From Parker 
and Haswell, after Ahlborn. ) 
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. 
