OKGAN OF SIGHT IN BIRDS. 137 



attachments of their anterior and posterior edges to the sclerotic 

 coat ; and by their being bound together by a tough ligamentous 

 substance, as it were the continuation of the sclerotic between 

 the edges that overlap each other. 



The cornea, fig. 57, c, possesses the same structure as in IMam- 

 malia, but differs with respect to form. When the posterior part 

 of the eye is compressed by the muscles, the humours are urged 

 forward and distend the cornea ; which, at that time, becomes 

 more prominent than in Mammalia ; and under such circum- 

 stances, the eye is in a state for perceiving near objects. When 

 the muscles are relaxed, the contents of the eyeball retire to the 

 posterior part, and the cornea becomes flatter: this is the con- 

 dition in which we find the eye of a dead bird, but we can have 

 no opportunity of perceiving it during life. It is only practised 

 for the purpose of rendering objects visible that are placed at an 

 extreme distance. From the well-known effects of form upon 

 refracting media, it must be presumed, that the cornea is least 

 convex when a bird which is soaring in the higher regions of the 

 air, and invisible to us, discerns its prey upon the earth ; its form 

 mil change as the bird descends A\4th unerring flight to the spot, 

 as is customary with many of the rapacious tribe. 



On reflecting the sclerotica from the choroid, 

 a grey substance is seen upon the fore part of 

 the latter, like a ring : it consists of fibres 

 showing, like those of the iris, the transverse 

 stride, and which serve to attach the choroid to 

 the sclerotic plates and contiguous margin of 

 the cornea. These fibres are regarded by the section of eyebaii, Faico. 

 anatomist, who first called attention to their xxxiv. 



muscular nature in Birds, as helping * to accommodate the eye to 

 the different distances of objects,' being supposed to act upon 

 the cornea in a manner analogous to that of the muscles of the 

 diaphragm upon its tendinous centre.^ 



The choroid coat is a loosely cellular and highly vascular mem- 

 brane, devoid of ' tapetum,' and copiously covered or saturated with 

 a black pigment. Opposite the bony circle the choroid separates 

 into two layers ; the external layer is the thinnest, and adheres at 

 first firmly to the sclerotica, after which it is produced freely 

 inwards to form, or be continuous with, the iris. 



The iris, fig. 57, ^, is delicate in its texture, which under the lens 

 appears composed of a fine network of interlacing fibres, but it is 

 remarkable for the activity and extent of its movements, which 



' XXXIV. p. 170. 



