192 LECTURE VIII. 



Anatomy to the olfactory nerves are to be understood of the crura rhin- 

 encephali. In the Lancelot the little ciliated olfactory sac {^fig. 46. ol.') 

 is brought into close contact with the rhinencephalic extremity of the 

 neural axis. When the olfactory lobe or ganglion, in other fishes, 

 is near the organ of smell, it sends off the nerves by numerous very 

 short fasciculi : this characteristic multiplicity of virtual origins of 

 the proper nerve is less conspicuous where the rliinencephalon is near 

 the rest of the brain ; but a careful analysis of the long olfactory 

 nerve in the Eel, the Ide, or the Roach, shows that it is a fasciculus 

 of filaments distinct from their origin. 



The optic nerves, like the eyes, are of large relative size in most 

 fishes : but where the organs of sight are small, the nerves are 

 slender, as in the Silurus : they are still more slender in the 

 Myxinoids, and they are scarcely discernible filameiits in the Am- 

 blyopsis. In the Plagiostomes, the Sturgeon, the Polypterus, and 

 the Lepidosiren, the optic nerves, traceable in part from the optic 

 lobes, closely adhere to the basis of the mesencephalon, from which 

 they seem to rise, anterior to the infundibulum, and are there con- 

 nected together by a short transverse commissure ; but they do 

 not cross each other. In ordinary Osseous Fishes the exterior 

 white fibres of the optic lobes converge to their under and anterior 

 part, to form the chief part of the origin of the optic nerves ; but 

 a portion of the origin may be traced through the septum opticum to 

 the cerebellum ; and in the Eel, the Gar-pike, and the Lump-fish, a 

 portion may be traced to the hypoaria : in the Cod some fibres of 

 the optic nerve are derived from both the hypoaria and the wall of 

 the third ventricle. The nerves are connected together at their 

 origins by a commissure ; but afterwards they cross one another 

 without interchange of fibres {Jig- 53. 2) : sometimes the right nerve 

 in its passage to the left eye passes under, sometimes over, the left 

 nerve * : rarely does one nerve perforate the other, as, e. g. in the 

 Herring. The nerves are flattened where they decussate. In most 

 Osseous Fishes the structure of the optic nerve is peculiar ; it consists 

 of a folded plate of membrane and neurine {fig- 57. a). The retina 

 is formed by the unfolding of the nerve ; and it would be a forced and 

 overstrained analogy to compare it with the ganglion of the olfactory 

 nerve (rliinencephalon), because this happens in some fishes to be close 

 to the nasal capsule. The optic nerve escapes, in Osseous Fishes, 

 either through the antei'ior fibrous wall of the cranium beneath the 

 orbito-sphenoid, or through a notch or a foramen in that bone. In 

 the Flounder one optic nerve is usually shorter than the other. In 



* I have seen both varieties in different individuals of Gadus morrhua. See 

 also i-xxv. ii. p. 203. 



