PISCES, TRUE FISH 



469 



Nervous System 



The central nervous system, composed of the brain and spinal cord, 

 is a little more developed than it is in the lamprey or shark. The 

 cerebral hemispheres are closely fused with the olfactory tracts which 

 extend by long tracts anteriorly to the olfactory bulbs (Fig. 256). The 

 diencepMlon, which is immediately posterior to the cerebrum, is cov- 

 ered dorsally by the large dome-shaped cerebellum (much smaller in 

 the perch). Extending dorsally from the roof of the diencephalon is 

 a slender, fingerlike pineal body (epiphysis) which is the vestige 

 of a third or median eye. Also partially covered by the cerebellum 

 is the midbrain which is divided into two rounded optic lobes. The 

 medulla oblongata lies just posterior to the cerebellum and is quite 



Fig. 257. 



OI_FACTORV TRACT 

 OPTIC NERVE C2) 

 CEREBRUM 



OCUL.OMOTOR Ni C35 



HYPOPHYSIS 

 OPTIC UOBES 



NFERIOR LOBES 

 TROCHL-EAR N. 0*5 

 TRIGEMINIES N. Ca3 

 AND FACIAL N. CT) 

 ABDUCENS Nt (6) 

 MEDUUl_A OBUONGATA 



AUDITORY N. fe) 

 GL_OSSOPHARYNGEA1_ N .(9) 



VAGUS N. <'lO'> 



-Ventral view of brain and cranial ner^^es of Ameiurus natalis Le Sueur. 

 (Drawn by Titus Evans from dissections by Rose Newman.) 



prominent, due to the large posterior lobes (tubercula acoustica) on 

 each of its anterolateral positions. Dorsally, between these lobes, is 

 a diamond-shaped slit which leads into the cavity of the brain. This 

 is the fourth ventricle. On the ventral side of the diencephalon is 

 the optic chiasma where the optic nerves meet, and behind this are 

 the inferior lobes with the stalklike infundibulum joining the glandu- 

 lar hypophysis. Two peculiarities of the brain of Ameim^us are the 

 large cerebellum and the large posterior lobes. There are ten pairs 

 of cranial nerves emerging from various levels of the brain. Three 

 of these have strictly sensory function, three are strictly motor in 

 function, and four have both sensory and motor function. The bull- 

 head has forty-one pairs of spinal nerves arising segmeutally from 



