XIV 



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 



ATut 



JVvl 



■Nv.8 



med-obL 



Fig. 211. — Dorsal (A) and ventral (B) views of tlie brain of Petromyzon marimis. 

 ck.pl.l, Anterior clioroid plexus forming tlie roof of tiie prosenceplialon and tlialam- 

 eneephalon ; 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, thalamencephalon ; 171/. infundibulum ; l.gn.hb, left ganglion 

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

 motor ; nv,5, trigeminal ; and nv.S, auditory nerves ; olf.l, olfactory lobes ; opt.l, 

 optic lobes ; pn, pineal organ ; r.gn.hh, 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 tlie brain in 

 Cyclostomes and Fishes will now be briefly dealt with. 



