The elasmobranch fishes . elasmobranchfish03dani Year: 1934 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.) , anterior lobe; , intermediate lobe; )., inferior lobe of brain; , inferior lobe of pituitary; inf., infundibulum; , superior lobe; , vascular sac of brain; //, III, second and th
The elasmobranch fishes . elasmobranchfish03dani Year: 1934 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.) , anterior lobe; , intermediate lobe; )., inferior lobe of brain; , inferior lobe of pituitary; inf., infundibulum; , superior lobe; , 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 () 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 (). 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 () 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 {, fig. Fig. 216. Transverse section through the 216) forms a swelling on the lateral medulla, MusteJ^. (From
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