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)

    67/142 (page 67)
    the wild species, from which these races are supposed to be derived, but the colour varies also individually, an occurrence almost unknown in wild specimens. In the Ungulata, notably the Horse, Ass, Mule, and Cow, the variation is far less marked than in the Carnivora. In the Carnivora, with their dull grey Tapetum cellulosum, the colour of the fundus depends entirely upon the retinal pigment, and this colour is easily susceptible to variation by domestication and interbreeding, whilst in the Ungulata the colour of the fundus is mainly determined by the Tapetum fibrosum which, being choroidal, remains practically unchanged, the altered retinal pigment determining but a slight change of hue. The Dog shows the greatest amount of variation, in conformity with the almost endless races into which this pliable genus (or perhaps several genera, i.e., Wolves, Foxes, and Jackals) has been modified by domestication and artificial selection. The tabulation of the ophthalmoscopic results, based upon a large number of Dogs, with due regard to their individual pedigrees, might enable us to obtain a reliable guide as to the purity of the breed of any given example. Nicolas and Fromaget* have classed the ophthalmoscopic colour of the Fundus oculi of the Horse according to the colour of the coat of the animal, but their results would probably be more conclusive if the examination were undertaken from the standpoint of racial difference. The influence of domestication, which is merely forced life under new—often unnatural—surroundings and conditions, is also well indicated by the frequent occurrence of myopia and astigmatism, which we find most prevalent in the Horse, and most marked in the Rabbit, especially if we compare the latter with the high degree of hypermetropia in the Hare. This myopic condition probably receives its explanation in the small dark hutches in which these Rodents are generally confined. Myopia is almost unknown in wild animals, but it occurs in wild specimens which have long been kept in captivity. XI. The Ophthalmoscopic Appearance of the Fundus oculi as bearing on Classification. Haeckel, in his ' Systematic Phylogenesis,'f remarks that the organisation and development of the eye in the vertebrates is, so far as their phylogenesis is concerned, of very limited interest. With the material at his disposal he could not have come to any other conclusion. Comparative ophthalmology is recent, and no attempt has yet been made to arrange the facts hitherto ascertained with a view to generalisation, and of testing their taxonomic and phylogenetic value. I hope that the observations * ' Ophtalmoscopie Vetfirinaire.' Paris, 1898. t Vide ' Systematische Phylogenie der Wirbelthiere,' Ernst Haeckel. Berlin, 1895, p. 127. K 2