. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 304 AVES. together with a tough ligamentous substance, which seems to be the continuation of the scle- rotic between the edges that overlap each other. The cornea possesses the same structure as in mammalia, but differs with respect to form. When the posterior part of the eye is com- pressed by the muscles, the humours are urged forwards and distend the cornea; which, at that time, becomes much more prominent in most birds than it is ever observed in mammalia ; and under such circumstances, the eye is in a state for perc
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 304 AVES. together with a tough ligamentous substance, which seems to be the continuation of the scle- rotic between the edges that overlap each other. The cornea possesses the same structure as in mammalia, but differs with respect to form. When the posterior part of the eye is com- pressed by the muscles, the humours are urged forwards and distend the cornea; which, at that time, becomes much more prominent in most birds than it is ever observed in mammalia ; and under such circumstances, the eye is in a state for perceiving near objects. When the muscles are quite relaxed, the contents of the eye-ball retire to the posterior part, and the cornea becomes flat or even depressed : this is the condition in which we always find the eye of a dead bird, but we can have no opportunity of perceiving it during life. It is only prac- tised for the purpose of rendering objects visi- ble that are placed at an extreme distance. From the well-known effects of form upon re- fracting media, it must be presumed, that the cornea possesses very little, if any, convexity, when a bird which is soaring in the higher re- gions of the air, and invisible to us, discerns its prey upon the earth, and descends with uner- ring flight to the spot, as is customary with many of the rapacious tribe. The degree of convexity of the cornea is also changed in birds by the action of muscular fibres especially appropriated to its motions. These were discovered by Crampton ; are dis- posed around the circumference of the cornea, and are attached to its internal layer; they draw back the cornea, in a manner analogous to the action of the muscles of the diaphragm upon its tendinous centre. The choroid coat re- sembles in its structure that of mammalia; it is copiously covered with a black pigment, similar to that in the human eye. Opposite the bony circle the choroid separates into two layers; the exter- nal layer is the thin- nest
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