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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    I. Palpebral Apparatus. Within the range of fishes included in the above list there can be found every form of provision for the setting and protection of the outer surface of the bulb, from the most simple and least developed to a complex and highly developed mechanism. Such a series might naturally be expected to be concurrent with an evolution from the simpler to the more complex type of fish, but such a parallelism in development cannot be held to be a real one, since in most cases the complex forms of palpebne are found in fishes classed within orders nearest the main line of phylogeny, whilst the simpler forms may be found in the more specialised types. This would appear to indicate that the variation of these accessory organs depends peculiarly upon the conditions of life of any particular family, which suggestion is confirmed by the variations of form found in closely allied species. 1. Palpebral Provision—None. There are some, but not many, species in which these negative conditions are found. The actual arrangement of the transition from the skin surrounding the orbit to the outer corneal layer is of two kinds. One in which the passage is by an uninterrupted and even layer of skin which is directly continuous with the transparent outer corneal layer. This arrangement necessarily restricts the movements of the eyeball within the narrowest limits. It is found well exemplified in the common and Conger Eels, also the Lamprey and Globe-fish; in the Conger its in- fiuence upon the bulbar muscles is marked. It is also found in a less degree in the commoner Eays. A second form is that in which the skin passes from the orbital margin to the outer corneal layer by a loose and flexible collar of skin, which allows of free bulbar motion in all the combinations of rotation and rectilinear motions, and also in some cases of projection and retraction. This arrangement can best be seen in the Pleuronectes, where the projection of the bulb allowed by this arrangement gives the eyes of these fishes an appearance not unlike a miniature conning-tower. A similar condition less well marked is found in the Angler. O