15& Mr Twining on Single Vision. 



previous observations, we have reason to conclude that the 

 structure assumed as the basis of their reasoning is not neces- 

 sary to the function of single vision. 



We must be careful how we attempt to employ the facts 

 which comparative anatomy affords, in explanation of the phe- 

 nomena of vision, as performed by the human subject ; for it 

 is reasonable to conclude, that the organ or instrument of sight 

 is constructed to accord with the medium in which the animal 

 lives, and that the nature and degree of vision in each animal 

 has that particular modification which is best adapted to the 

 animal's wants and habits of life. We know that double vision 

 has occurred in man, when, by accident, an aperture has been 

 formed in the iris, besides the natural pupil, so that in fact there 

 were two pupils.* But we have as yet no satisfactory account 

 of the functions of vision in a species of fish, (the Cobitis Ana- 

 bleps,) whose eye is furnished with two pupils. The cornea is 

 opaque hi the mustyphlus, the Murena ccecilia, and the Gastro- 

 branchus cozcus. Both the cornea and aqueous humour are want- 

 ing in the Sepia, in which there is only a thin membrane over 

 the lens. In Lawrence's work on comparative anatomy, we are 

 referred to p. 341 of the Biology qfTreviranus, who states, that 

 the retina in the mole is formed by an expansion of a branch of 

 the superior maxillary division of the fifth pair of nerves.f 

 Magendie has observed, that, when birds are blind of one eye 

 from the destruction of the cornea,' the optic nerve of the blind 

 eye is wasted, and this atrophia extends to the optic thalamus 

 of the opposite side ; but he did not find the same occurrence 

 to take place in mammiferae. 



These and other varieties of structure, which comparative 



* See a case of diplopia from double pupil by Ragellini, in the Act. 

 Hafn. No. 1. A. 27 .—-Also a similar case from two accidental apertures in 

 operating for artificial pupil, in p. 66 of Sir W. Adams's work on artificial 

 pupil, published in 1814. — In page 231, of Saunders's Treatise on the Eye, 

 (1816 edition) is a case of double vision, arising from two apertures in the 

 opaque lens, in consequence of unequal absorption after the anterior ope- 

 ration for cataract. 



+ Magendie and Demoulins ascertained, that the mole has no nerve cor- 

 responding to the optic of other animals, and that there exists no foramen 

 opticum in the sphenoid bone for transmission of such a nerve ; authenti- 

 cating fully the observation of Treviranus. 



