236 



THE ELASMOBRANCH FISHES 



myelencephalon, the fourth ventricle above mentioned, with the cavity of 

 the diencephalon, the third ventricle. In the wall of the diencephalon is the 

 optic thalamns, and, in its roof, the habennlar ganglion from which the pineal 



Fig. 215. The pituitary, Squohis siiclUi, side view. (Marie Carlson, orig.) 



a.L, anterior lobe; i.l., intermediate lobe; i.l.l)., inferior lobe of brain; i.l.h., inferior lobe 

 of pituitary; inf., infundibulum ; s.L, superior lobe; v.s., vascular sac of brain; //, III, 

 second and third nerves. 



stalk arises. In the floor of the diencephalon is the infundibulum (in.). The 

 cavities in the right and left lobes of the telencephalon are the lateral 

 ventricles. 



Figure 216 (Houser, 1901) is of a transverse section through the medulla of 

 Mustelus to show something of its finer structure. In its median ventral mass 

 lies the abducens nucleus, the fibers of which form the abducens or sixth 

 cranial nerve. Around the nucleus of the sixth nerve the tract cells are scat- 

 tered. These cells are of interest in that they are exceeded in size only by the 

 cells in the roof nucleus of the mesencephalon (see fig. 206) . The lobes of the 

 vagus (l.v.) are made up of masses of cells which receive visceral sensory 

 fibers from the seventh, ninth, and tenth cranial nerves. Some of the axones 



from the cells of these lobes pass down- 

 ward a short distance to the viscero- 

 motor nucleus (vm.n.). The cells in this 

 large nucleus give rise to the motor 

 fibers of the fifth, seventh, ninth, and 

 tenth nerves. 



Houser states that the general cuta- 

 neous nucleus (g.c.n.) is the terminus 

 for the somatic sensory fibers of the 

 fifth, ninth, and tenth nerves, but ac- 

 cording to Norris and Hughes (1920) 

 the ninth and tenth nerves contain no 

 somatic sensory elements. 



The tuberculum acusticum {t.a., fig. 



Fig. 216. Transverse section through the 216) forms a swelling on the lateral 



medulla, MusteJ^. (From Houser.) ^^^^ ^^ ^j^^ ^^^^^^j^ ventricle (r.^) and 



flm., median longitudinal bundles; f.r., 



formatio-reticularis ; l.v., lobes of vagus; is principally a center for fibers from 



g.c.n., general cutaneous nucleus; t.a., ^j^^ j^^g^,^^ fj^^g ^^ ^^^^ f^,^^^ ^^le 



tuberculum acusticum; rm.?i., visceromotor _ 



nucleus ; v.\ fourth ventricle. internal ear. 



