Gasserian ganglion 

 saccus dorsalis 



pineal (dashed outline) 

 superficial ophthalmic VII 



.dorsal line X 

 lateral line X 



buccal branch VII 



pituitary/ 



palatine branch VII 



geniculate ganglion 

 hyomandibulor branch VII 



glossopharyngeal ganglion 



Figure 13-11. Roots ot cranial nerves of Amia. (After Norris, 1925) 



hypoglossal nerve {XII) 



The infundibulum is attached below to the pituitary. In the 

 region just above the saccus vasculosus there are distinct, 

 bilateral inferior lobes. 



Behind the diencephalon the optic lobes are large bilateral 

 structures. The floor and the sidewalls of the mesencephalon 

 are thick as in the other groups. The cerebellum is large 

 and has a small cavity (metencoel) extending upward into it 

 and into its various lobes. Posterolaterally there are folded 

 restiform bodies which project forward. There is no valvula 

 cerebellae in this group. 



The myelencephalon has a large chorioid plexus in its 

 roof with digitations extending down into the fourth ven- 

 tricle; the lateral walls and floor are not as well developed 

 as in the teleosts. 



Holocephalan The brain of Hydrolagus agrees in detail 

 with that of Squalus differing in that the diencephalon (and 

 part of the telencephalon) is much elongated (Figure 13-13). 

 This modification of the brain is related to the large eyes, 

 whose sockets are separated only by an interorbital septum. 

 The optic lobes are small and the cerebellum is not dis- 

 tinctly divided into bilateral halves. The lateral-line lobes 

 are larger and extend further back. The saccus vasculosus is 

 comparable. 



The chimaerid, like the shark (Figure 13-14), has the 

 usual ten cranial nerves, and the number of branches of 

 these is less than in the actinopterygian. The general distribu- 

 tion of these nerves is similar to that in other fishes with the 

 seventh, ninth, and tenth supplying the lateral-line organs. 

 The autonomic system is like that observed in the mammal. 



Cyclostomes 



lamprey In the lamprey (Figure 13-15) the olfactory 

 nerves are paired and relatively short. They extend back 

 from the walls of the midline nasal capsule to enter the 

 medial anterior aspect of the olfactory bulbs. These bulbs 

 are the larger part of the telencephalon. Behind each is a 

 small cerebral lobe. The olfactory bulbs and cerebral lobes 

 have small ventricles which connect with the medial third 

 ventricle, a good part of which lies in the telencephalon. 



The diencephalon has a membranous roof which is evag- 

 inated outward as a large saccus dorsalis. Anteriorly to the 

 saccus is a small paraphysis; on top of the saccus rests a 

 parietal organ and above this a pineal organ. Both of these 

 organs are developed as photoreceptors. The parietal organ 

 has a nerve extending down to the left habenula; the nerve 

 of the pineal organ extends down and back on the right 

 side to the posterior commissure. The anterior end of the 

 diencephalon is not marked by a velum transversum. 



Ventrally the optic nerves cross without exchange or in- 

 terlacing of fibers. The hypothalamus has small inferior 

 lobes and an indistinctly differentiated "saccus vasculosus." 

 The floor of the infundibulum, the pars nervosa, is attached 

 to the underlying anterior lobe of the pituitary. 



The mesencephalon has bilateral optic lobes between 

 which is a membranous, evaginated chorioid plexus. This 

 plexus is fused to the plexus overlying the fourth ventricle 

 in the myelencephalon. A small transverse cerebellum and 

 a tapered myelencephalon, much like those of the amphib- 

 ian, lie behind the optic lobes. 



THE CONDUCTING AND INTEGRATING SYSTEM 



395 



