Observations on the comparative microscopic anatomy of the cornea of vertebrates / by W. H. Lightbody.

  • Lightbody, W. H.
Date:
[1867]
Catalogue details

Licence: Public Domain Mark

Credit: Observations on the comparative microscopic anatomy of the cornea of vertebrates / by W. H. Lightbody. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    processes, either entirely fibrous, or partly fibrous and partly com- posed of the same structureless tissue as the membrane itself, are sent off from its free surface to the iris, while the greater part goes on and ends as in other animals. There is a remarkable exception to this description in the seal, in which animal at the point of transi- tion of the sclerotic into cornea, a bed of tissue closely resembling tendon on section is interposed between the cornea and the elastic lamina: this bed is thick towards the sclerotic and ends in a rounded form, towards the cornea it rapidly thins and ends in a sharp edge. The elastic lamina sends into this structure five or six digitations, which are sometimes branched and end in slightly clubbed extremi- ties, while a small portion passes on to the areolar tissue in front of the ciliary muscle. The function usually ascribed to this membrane to maintain the proper curve of the cornea is, I think, an insufficient explanation of its use; for I do not know that it ever forms a -^th of the thickness of the cornea, sometimes -g^jth, or even less; for example, in the seal this membrane just inside the edge of the cornea is only 4oVotn °f an inch, while in the centre it thins off to 12j00tn of an inch. Now the cornea of the seal is rather, though not excessively thick. From the habits of the animal it will have to support very sudden changes of external pressure: if then the preservation of the curvature of the cornea depended on this layer, we should expect to find it under such circumstances unusually thick, but it is just the opposite: on the other hand, in the sheep and horse, animals both having very thick cornese, well able to maintain their own curvature, especially as they have no abrupt changes of external pressure to undergo, this mem- brane is exceedingly thick. The means by which the curve of the cornea is maintained, I believe to be the tension of the eyeball, kept up by the due secretion of aqueous and vitreous humours. Another proof that the development of this membrane is not dependent on the curve of the cornea, may be found in the com- parison of the corneas of the rabbit and guinea-pig: they are both about the same size and thickness, that of the guinea-pig being rather the thinner; they are also about the same curvature, the rabbit being rather the flatter; yet Descemet's membrane in the rabbit is joVo^h of an inch thick, in the guinea-pig it is only -^^th. I think that it may have two principal uses; lstly, preventing the too rapid absorption of the aqueous humour by the cornea; 2nd]y, acting as a tendon to the ciliary muscle. When I say that it may serve as a tendon to the ciliary muscle, I do not mean that the curve of the cornea is at all altered; tense as is the eyeball in a normal condition, this could hardly be effected by