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Dr. R. J. Lee on the Sense of Sight in Birds. [May 16, 



The eye of the Eagle Owl presents in the most striking degree the 

 peculiar characters of the class to which it belongs. The first of these 

 are its shape and size, too well known to require description, adapted as 

 they are to the very shallow cavities of the orbits. 



In the Egyptian Vulture the pyramidal shape of the eye is less remark- 

 able, and a slight approach is observable in it to the spherical globe. In 

 the Buzzard this is still more marked, and the eye resembles as much the 

 eye of the Pigeon as it does that of the Eagle Owl. 



In examination of specimens which have been preserved in spirit, it is 

 necessary to restore the pliancy of the tissues of the ciliary muscle by 

 allowing them to remain in water for some days ; and I may observe that 

 as this condition must be obtained in order to make satisfactory prepara- 

 tions, the method of using solutions of chromic acid or the bichromate of 

 potash to enable the anatomist to make sections is not to be recommended, 

 if the object be to ascertain the dimensions of the muscle and the elasticity 

 of the ligament, which will be presently described. It need hardly be 

 stated that the best mode of treating the eye is to freeze it and then make 

 sections. 



The strong plates of bone which exist in the sclerotic of birds preserve 

 the shape of the eye sufficiently well to allow of the dimensions being 

 ascertained after it has been preserved in spirit. 



In the Eagle Owl the dimensions are as follow : — - 



Antero-posterior length 1 ^ 



Lateral diameter of lens -fy 



Antero-posterior diameter of lens \ 



The shape of the lens does not appear to be altered by the action of 

 alcohol ; but the size is diminished, and the measurements just stated are 

 less than they would be found to be if the lens had been perfectly fresh. 



The eye is first to be divided into equal halves by cutting through the 

 sclerotic, choroid, cornea, and iris. We may regard the sclerotic as a 

 hollow case enclosing a sphere, of which the choroid is the proper cover- 

 ing, and which sphere is attached to its case by tissues of highly elastic 

 and muscular properties, by which a certain amount of movement is capable 

 of being effected in the parts on which the formation of the image depends. 

 It is to be observed, however, that the posterior surface of the choroid is kept 

 in close apposition with the inner and posterior surface of the sclerotic, so 

 that movement of the anterior parts is not communicated to that part on 

 which the optic nerve is expanded. In the eye of the Eagle Owl these 

 conditions are obtained in the following manner. 



The whole of the posterior surface of the choroid which corresponds to 

 the optic disk is kept in close apposition to the sclerotic by the direct 

 attachment of the circumference of the part immediately beneath the 



Diameter of cornea . . . 

 Diameter of base of eye 



