16 bird's-eye views. 



this reason known as the "vitreous humor." There is much 

 less aqueous than vitreous, because the anterior chamber is 

 much the smaller of the two ; but birds have more of the 

 former, compared with the quantity of the latter, than 

 mammals, because the size and convexity of the cornea is 

 relatively greater. The aqueous humor is enclosed in a 

 very delicate simple membrane, that cannot be demonstra- 

 ted without difficulty. The vitreous is contained in a more 

 palpable, as well as complex membrane — the hyaloid (e) 

 — which, besides lining the interior of all the back part of 

 the eye, and enclosing the lens as already described, sends 

 thin lamina?, or layers, all through the vitreous humor, form- 

 ing partitions that serve to steady the glassy waters. 



We may next turn our attention to the optic nerve (a) 

 that presents itself in the all-important character of the "soul 

 of the eye." It has many peculiarities in birds ; among them 

 one that constitutes the most characteristic feature of the eye 

 of these creatures. In mammals, as a general rule, the nerve 

 is a smooth cylinder that comes straight to the sclerotic, near 

 the middle behind, penetrates straight through the coats of the 

 eyeball, and then spreads out on all sides to form a disk on 

 the inside of the back of the eye. This circular saucer-like 

 expansion is the retina — ^,the sensitive nervous plate, or mir- 

 ror, upon which images of things viewed are photographed, 

 to be transmitted along the nerve to the brain, and there 

 "perceived." Suppose the optic nerve to be an umbrella- 

 handle, the retina Avould then be the umbrella, blown inside 

 out by the wind. In birds the nerve acts very differently. 

 In the first place, though it is cylindrical, it is not smooth ; 

 it has leno-thwise folds and ridges. It is like a fluted column. 

 It comes ol)liquely towards the eye, which it strikes at a 

 point eccentric from the axis of the ball ; and then, instead 

 of at once piercing all the sclerotic coats, and expanding 

 into a concavo-convex disk, it tapers gradually to a fine 

 point. This elongated extremity runs still obliquely, down- 

 wards and forwards, in a deep groove in the sclerotic, that 



