There are, in fishes, several ophthalmic nerves between which it is necessary to carefully distinguish. While their exact inter-homologies cannot as yet be definitely affirmed, certain very probable homologies can certainly be arrived at.
The portio ophthalmici profundi of bony ganoids, and the ramus ophthalmicus profundus and ramus ophthalmicus superficialis trigemini of elasmobranchs, are general sensory cutaneous nerves, and probably contain all of those general sensory elements that belong to the ophthalmic nerves of vertebrates.
The portio ophthalmici profundi of ganoids is the homologue either of the ramus ophthalmicus superficialis trigemini of elasmobranch Sj or of a frontal branch of the ramus profundus that has, in certain selachians (Læmargus), the position of a superficial ophthalmic nerve; or it is the homologue of both those nerves of elasmobranchs. In the higher animals this portio profundi becomes the frontal branch of the ophthalmic nerve.
The ramus ophthalmicus profundus of elasmobranchs and of Polypterus, and that nerve alone, is the homologue of the ramus nasalis or naso-ciliaris of higher animals. This nerve and the portio ophthalmici profundi vary inversely in importance.
The ramus ophthalmicus superficialis trigemini of Ainia is a communis nerve, and is probably the homologue of that part of the ophthalmicus superficialis facialis of Acipenser and elasmobranchs that is derived from the so-called dorsal root of Trigeminus II. This latter root arises from the lobus trigemini of the latter fishes, is considered by certain authors as a lateral sensory root, and is called by them the dorsal root of the ramus opthalmicus superficialis facialis.
The ramus ophthalmicus superficialis facialis of Ami a is a lateral sensory nerve, and has its probable homologue in that part only of the ophthalmicus superficialis facialis of Acipenser and elasmobranchs that is derived from the so-called ventral root of Trigeminus II. This root arises, in all fishes, from the tuberculum acusticum.
The so-called ramus ophthalmicus superficialis trigemini of teleosts is the ramus ophthalmicus superficialis trigemini of Amia plus the portio ophthalmici profundi of that fish, these two components being found in varying proportions.
The ramns ophthalmicus superficialis of selachians probably always contains the same three components, or portiones, that are found in the superficial ophthalmic nerve of Amia, but the portio ophthalmici profundi, called in selachians the portio trigemini, is usually small and may be reduced to a few fibres only (Rays, Ewart). The portio trigemini of Amia is represented in selachians, as stated above, by those fibres of the so-called portio facialis that arise from the lobus trigemini; that is, by the fibres that form the so-called dorsal root of Trigeminus II.
The terminal buds of ganoids and teleosts, the nerve-sacs of Acipenser, and the ampullae of selachians, are in all probability homologous structures.
- Copyright © 1901 by the Company of Biologists Ltd.