THE ELASMOBRANCH FISHES 



239 



varies greatly. In some t'orins the nerve is so sliort as hardly to be observed 

 from the outside. This is observable in types like Heptanchus (fig. 200a), 

 Galeus. and Scifiiiniis. In Seoliodon (fig. 211a) the olfactory bulbs are farther 

 sei)arated from the nasal ei)ithelium so that the two divisions of the nerve (I) 

 are distinct. In Echinoyhinus the nerves are of an extreme length, rarely met 

 with in any other Elasmobranch. 



A sagittal section through the anterior part of the olfactory bulb would 

 show how the fibers come to masses of cells (glomeruli) in the bulb from the 

 olfactory membrane. These are arranged in two 

 bundles. One of these is median and ventral in posi- 

 tion; the other is lateral. The fibers from the cells in 

 the glomeruli of the olfactory bulb are continued to 

 the olfactory lobe of the brain by the olfactory tracts 

 (ol.t.). These tracts similarly vary in extent through 

 extremes shown in Eckinorhinus and Hexafichus. In 

 the former the bulb and the lobe are practically con- 

 tinuous; in the latter the tracts are greatly drawn out. 



Accompanying the olfactory nerve is the terminal 

 nerve {tn., fig. 211) studied in detail by Locy (1905) . 

 This nerve in Acanthias as a type {t.n., fig. 172a) 

 ends in the thickened lamina terminalis. The right 

 and left nerves at the entrance of the brain would 

 thus be separated by the median fissure. Through 

 growth of the brain and fusion of the median olfac- 

 tory nuclei it so happens that the terminal nerve 

 instead of passing directly forward may arise dor- 

 sail}' or ventrally. Those forms in which the nerve 

 has a connection with the brain dorsal to the neuro- 

 pore are : Raja, Trijgon, M[jliobatis. In others the 

 connection is more nearly dorsal than ventral {Acan- 

 thias, Squatina, Hexanchiis) . In still others the con- 

 nection is ventral, as in Galeus, Scoliodon (fig. 211a) , 



Scyllium, Pristiurus, and Carcharias littoralls. In all the al)ove forms the 

 nerve passes forward as a double or triple strand on which is a ganglion (or 

 a series of ganglia). The nerves join the olfactory fila as in Heptanchus and 

 go to the mucous lining of the olfactory cup. 



The optic or second nerve (77, figs. 210 and 213a) is sufficiently similar in 

 the different forms to need but little description. It arises from cells in the 

 retina and its fibers cross to form the optic chiasma. They then extend to, and 

 terminate in, the diencephalon and mesencephalon. 



The oculomotor nerve (777, figs. 200a and 200b) arises in a nucleus under 

 the aqueduct of Sylvius and leaves the brain stem near the middle line of the 

 mesencephalon. It enters tlie orbit and gives otf a dorsal branch to the superior 

 rectus and the anterior rectus and a ventral branch which divides into an an- 



-md.V. 



Fig. 219. Ganglia of tri- 

 geminal (F) and facial 

 ( VII) nerve, median view, 

 Chlamydosclachus. ( From 

 Mrs. Hawkes.) 



hu.VII, buccal division 

 of seventh nerve; g., gas- 

 serian ganglion; md.F, 

 mandiljular nerve; tiix.V, 

 maxilliary l)rancli of fifth 

 nerve; op.V, ophthalmicus 

 profundus nerve; os.V, 

 oi)hthalmieus superficialis 

 of fifth; os.ril, ophthal- 

 micus superficialis of 

 seventh. 



