214 PISCES. 



possesses a central canal. Occasionally it is very short, as in those 

 Fishes, Orthagoriscus, Lophius piscatorius, that are provided with a 

 short truncate vertebral column. The spinal cord, however, is usually 

 very long, and exhibits generally a faint enlargement at the spot 

 whence the nerves arise that supply the extremities. 



As regards the Cerebral nerves, from ten to eleven pairs are dis- 

 tinguishable in the Osseous and true Cartilaginous Fishes, and they 

 agree in the relations of their origin and course with those of the 

 higher Vertebrata and Man. 



The olfactory nerves are for the most part slender in the Osseous 

 Fishes, and arise frequently by several roots (3 to 5), from the olfac- 

 tory tubercle of which they appear to be the immediate continuation. 

 In the true Cartilaginous Fishes they are often thick and short, as in 

 Scy Ilium, or, as in the Rays, frequently long and slender ; they arise 

 from the hemispheres, being in connexion with their ventricular cavity 

 when present, and form at their extremity very large ganglionic en- 

 largements, comparable to the clavate extremities of the olfactory 

 nerves in Man and Mammalia. 



The optic nerve arises from the mesocerebrum (t. e. the lobi optici 

 and that narrow part of the brain that surrounds the third ventricle 

 in the Plagiostomi) : in the Osseous Fishes each optic nerves takes 

 its chief origin from the optic lobe of the opposite side ; both nerves 

 thus cross each other completely, so that the left passes to the right 

 eye, and vice versa; in the Herring, indeed (Clupea harengus), the 

 optic nerve of the right eye perforates that of the left, passing by 

 means of a slit throngh its fibres, without forming a chiasma. The 

 optic nerves form band-like strips, folded longitudinally. In the 

 Plagiostomi and the Sturgeon the optic nerves are united by a true 

 chiasma, some fasciculi only crossing each other. 



The ocular nerves, namely, the third, fourth and sixth pair, ex- 

 hibit similar relations of origin and course to those of Man. The 

 oculo-motor nerve issues of large size from above and behind the in- 

 ferior lobes ; the nervus patheticus from between the optic lobes and 

 the cerebellum, and passes to the trochlear muscle. The delicate 

 abducens nerve arises distinctly by two roots at some distance be- 

 hind the inferior lobes from the basal surface of the medulla oblongata, 

 and passes to the rectus externus muscle. The ciliary branch of the 

 oculo-motor nerve appears to be absent. 



The fifth pair (n. trigeminus] is in the Cartilaginous Fishes, at 

 least in the Rays, the most largely developed nerve, but frequently, 

 as in the Osseous Fishes, yields in respect of size and extent to the 



