NERVOUS SYSTEM 



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 



tfut 



med-obl ~ 



FIG. 211. Dorsal (A) and ventral (B) views of the brain of Petromyzon marinus. 

 ch.pl.l, 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 ; cr. crb, crura cerebri ; crb, cerebellum ; crb. h, cerebral hemi- 

 spheres ; dien, thalamenoephalon ; inf. infundibulum ; l.gn.hb, left ganglion 

 habenulae ; med.obl, medulla oblongata ; nv.l, olfactory ; nv.2, optic ; nv.3, oculo- 

 motor ; 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.) 



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. 



