The palpebral and oculomotor apparatus in fishes : observations on morphology and development / by N. Bishop Harman.

  • Harman, Nathaniel Bishop, 1869-1945.
Date:
[1899]
Catalogue details

Licence: Public Domain Mark

Credit: The palpebral and oculomotor apparatus in fishes : observations on morphology and development / by N. Bishop Harman. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    2. Palpebral Margins and Conjunctival Fornices defined. This form is the one found in the majority of the teleostean fishes. The skin passes from the circular orbital margin towards the centre of the cornea, and after extending thereto in a greater •or lesser degree, recurves deeply, forming a palpebral margin. The depression formed by the continuance of the thin inner or conjunctival layer on to the bulb to blend with the outer corneal layer, constitutes the conjunctival fornix. This arrange- ment, other things being equal, allows of great freedom of rotation and rectilinear movements, according to the depth of tlie conjunctival fornices. The depth to which the folds extend may differ in the lower and upper parts, but since the angles of the eye are rarely defined, the passage from superior to inferior fornices is uninterrupted. The circular palpebral margin is in life upon a level with, or makes a gentle slope towards, the flat corneal surface, so pro- viding for the least resistance to water friction during pro- gression. Such palpebral margins, however, afford but little protection to the eye since the greater part of the cornea remains exposed, but in some species, e.g. Sturgeon, the margin is sufficiently wide and of dense tissue, with calcified epidermis, that it makes an efficient protection to the sclero-corneal junction. Of the condition in which the margin and fornices are small and shallow, the Torpedo (inferior fornix only), Chimsera, Percli and Perch-pike, Gurnard, Grey Mullet, Stickleback, Cod, Had- dock, Hake, Carp, Poach and Eudd, and the little Pipe-fish, may he taken as examples ; and for the broader margins and deeper form of fornices the Sharks, Sturgeon, Bass, Eed Mullet, Bream (sea and fresh), Mackerels, Tunny-fish, Whiting, Ling, Tench, Pike, Salmon, and the Herring. These examples do not allow of any classification into families or according to the habitat of tlie fish as do other structural variations, for differences are found between closely allied members of the same family, e.g. amongst the Cods. Thus in Cod, Haddock, and Hake the margins are narrow and the fornices shallow, whilst in Whiting and Ling the margins are fairly broad and the fornices deep.