610 AVES BIRDS. 



in broad gyrations through the sky, are scanning from that height 

 the surface of the ground, and looking out for mice or other little 

 animals on which to feed : but when the prey is seen, and the bird, 

 shooting down with the rapidity of a thunderbolt, stoops upon 

 the quarry, it must obviously be indispensable that it should 

 see with equal clearness and distinctness when close to its victim, 

 as it did when far remote; and to enable it to do this special 

 provisions have been made in the structure of the eye-ball. 



A glance at figure 279, exhibiting a section of the eye of 

 an Owl, will show the anatomist that in its general composition 

 the organ is similar to that of Man. The sclerotic and the 

 choroid tunics present the same arrangement, the transparent hu- 

 mours of the eye occupy the same relative positions, and the iris 

 and ciliary folds exist as in the human subject. Descending from 

 generalities, however, he will find many points in the organization 

 of a bmTs eye eminently deserving separate examination, and it is 

 to these we would specially invite his notice. First, the shape of 

 the eye-ball is peculiar : it is not spherical, as in man, nor flattened 

 anteriorly, as in fishes and aquatic reptiles ; but, on the contrary, 

 the cornea is rendered extremely prominent, and the antero-pos- 

 terior axis of the eye considerably lengthened. This is remark- 

 ably exemplified in the Owl ; in which bird, as Dr. Macartney* 

 pointed out, such is the disproportion between the anterior and 

 posterior spheres of the eye, that the axis of the anterior portion 

 is twice as great as that of the other. The obvious consequence 

 of this figure of the globe of the eye is to allow room for a greater 

 proportion of aqueous fluid, and for the removal of the crystalline 

 lens from the seat of sensation, and thus produce a greater con- 

 vergence of the rays of light, by which the animal is enabled to 

 discern the objects placed near it, and to see with a weaker light ; 

 and hence Owls, which require this sort of vision so much, possess 

 the structure fitted to effect it in so remarkable a degree. 



(680.) But it is evident, that, in order to retain this conical 

 shape of the eye-ball, some further mechanical arrangements are ne- 

 cessary, which in the spherical form of the human eye are not requi- 

 site. In Fishes, where the eye-ball is constructed upon entirely 

 opposite principles, being compressed anteriorly, cartilaginous sup- 

 ports are found imbedded in the sclerotic tunic, which, in some 

 cases, is absolutely ossified into a bony cup. In many Reptiles 

 the same end is obtained by placing a circle of bony plates around 



* Rees's Cyclopaedia, art. Birds. 



