Contributions to the comparative anatomy of the mammalian eye : chiefly based on ophthalmic examination / by George Lindsay Johnson.

  • Johnson, G. Lindsay (George Lindsay), 1853-1943.
Date:
[1900]
Catalogue details

Licence: In copyright

Credit: Contributions to the comparative anatomy of the mammalian eye : chiefly based on ophthalmic examination / by George Lindsay Johnson. 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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    Some of the normal conditions observable in certain animals closely resemble those which we find in Man as congenital defects or vestigial relics. 1. Membrana nictitans. A fully developed nictitating membrane, active enough to sweep the whole cornea, exists only in the Ungulata, and not even throughout this order. In the Carnivora and Marsupials it is much less developed, whilst throughout the Primates, Insectivora, Rodents, Edentata, and Echidna it is still more reduced, and with rare exceptions entirely without movement. The primary use of this third lid, viz., that of cleaning the corneal surface, is lost within the class Mammalia, and seems to serve chiefly to protect the eye in the animals which graze and poke their heads down into the long and sharp grass. 2. The retractor muscle of the eyeball is of frequent occurrence, chiefly in Marsupials, Edentates, Rodents, and Insectivora, i.e., in the lower orders of Mammals. 3. Opaque nerve fibres. All stages of opacity occur congenitally in Man, and are to be found normally throughout the Mammalia. Opaque nerve fibres are most marked in some of the Rodents and Marsupials. 4. Physiological Cup and congenital discoloration of the disc frequently occur in Man. An appearance similar to the physiological cup occurs in all Felidse, and in a considerable number of the other Carnivora ; also in the Flying Squirrels and some of the other Rodents. White and grey discs occur normally in a number of animals widely separated in classifica- tion, such as the Skunk, Rhinoceros, Porcupine, Armadillo, and Echidna. 5. Structures protruding from the disc into the corpus vitreum. A. Persistent hyaline artery. This congenital defect in Man is found as a normal condition in nearly all the Ruminants, and in a large number of Rodents. B. Vestiges of a pecten. In some of the Rodents, more especially in all the Agoutis, a button-shaped vascular pigmented rudimentary pecten protrudes from the disc into the vitreous. • It is remarkable to find in the Mammalia a relic of this Sauropsidan organ. In a number of Marsupials vascular protuberances from the disc into the vitreous occur in different forms. 6. Colobomata. The jjapillary coloboma (Fuchs' Coloboma) has its analogy in a white or coloured scleral ring, which is normally met with in a large number of animals. 7. Retinitis pigmentosa. In the Galagos and Lorides a spreading of pigment occurs circumf'eren- tially in the retina which greatly resembles Retinitis pigmentosa. If these nocturnal animals are exposed for prolonged periods to daylight, the pigment advances con- centrically, similar to the manner in which it progresses in Man, so that the animals gradually go blind. 8. Visible choroidal vessels and stippled fundus. Visible choroidal vessels * occur in most of the Simise below Hylobates, and in a number of the other orders. They are most marked in the Macropodicke and some of the other Marsupials, which present the appearance observable in the extreme cases of the analogous congenital defect in Man. Stippled fundi are found in the feline Douroucouli and in the Lemurs, an appearance occasionally met with in Man. 9. Ectropion of the Uvea. In a number of the Ungulates which have large oval pupils, either pigmented excrescences or a specialised development of the elastic fibres of the iris are met with, and these evidently serve to screen the eye against B 2