THH DEVBLOPMBNT OE BIBDS. 223 



all Birds are provided with large and efficient eyes. Their eye- 

 balls are generally longer from within outwards than those of 

 other vertebrate animals, and their crystalline lens is but little 

 convex. The whole eye is shortest and the lens least flat in 

 aquatic Birds and longest in the Owls. 



The sclerotic of birds is not only dense but contains bony plates 

 which overlap each other and by their contraction protrude the 

 aqueous humour — which is very abundant — and so render the 

 cornea more convex. An organ called the marsupium or pecten 

 is a vascular membrane which projects into the vitreous humour 

 along a line extending from near the entrance of the optic nerve 

 to the lens. It seems that this organ can be distended and then 

 must help to push the lens forwards. These various telescopic 

 arrangements facilitate rapid changes from very long to very 

 short sight. They are most needful for such active creatures 

 as Birds. A Hawk wUl suddenly descend a quarter of a mile 

 and probably can keep a creature it intends to prey on in focus 

 all the time of its descent. The Bird's eye is indeed the most 

 perfect of all. 



The nictitating membrane is drawn out oyer the eye by a 

 muscle which arises from the lower part of the inner side of the 

 sclerotic, and thence its tendon winds round the optic nerve and 

 passes over the eyeball to be inserted into the third eyelid. By 

 its contraction it would compress the optic nerve and so impair 

 sight, but that it passes through a tendinous sheath of a quad- 

 rate muscle which comes from the back of the sclerotic. When, 

 then, the winding muscle acts, the quadrate muscle acts at the 

 same time, and draws the tendon away from the optic nerve. 



The lower eyelid is more moveable than in Man and Mammals, 

 having its own depressor muscle, and contains a small car- 

 tilage. 



Two glands secrete fluid to lubricate the eyeball. One of 

 these, the Harderian gland, lies at the inner angle of the eye. 

 The other, the lachrymal gland, lies, as with us, at its outer 

 angle. 



The Detelopment os Bieds. 



It would be quite beside the purpose of this work to describe 

 in detail the very complicated process by which the germ of a 

 Bird transforms itself gradually into the structure of the adult. 

 Our object, we believe, will be completely attained by a brief 



