STKUCTTJKE OF THE EYE. 733 



sight. The Fish, immersed in a dense medium, can see but to a very 

 limited distance around it ; and the sphericity of the crystalline lens, 

 with the consequent contracted antero-posterior diameter of the eye- 

 ball, at once testifies how small is the sphere of vision commanded by 

 the finny tribes. The Bird, on the contrary, dwelling in the thin air, 

 and not unfrequently soaring into regions where that air is still further 

 rarefied, must survey a horizon even more extensive than that enjoyed 

 by the terrestrial Mammal ; while, from the rapid movements of the 

 feathered races, it becomes absolutely requisite that the focus of the 

 eye shall continually vary between the extremes of long- and short- 

 sighted vision. The birds of prey, as they fan the air at an altitude 

 which places them almost beyond the reach of human sight, or sail 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 as much 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 eyeball. 



(2079.) A glance at fig. 365, 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 humours of the eye occupy the 



same relative positions, and 



,, . . , .,. r ,, ,, . , Fig. 364. 



the iris and ciliary folds exist, 



as in the human subject. De- 

 scending from generalities, 

 however, he will find many 

 points in the organization of 

 a bird's eye eminently deserv- 

 ing separate examination; and 

 it is to these we would speci- 

 ally invite his notice. First, 

 the shape of the eyeball is 

 peculiar : it is not spherical, 

 as in Man, nor flattened an- 

 teriorly, as in Fishes and 



J) m Eye of the Owl. 



aquatic Reptiles ; but, on the 



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

 posterior axis of the eye considerably lengthened. This is remarkably 

 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 



* Rees's Cyclopaedia, art. BIRDS. 



