SEC. IV ANATOMY OF BIRDS 269 



through a pulley which changes its line of traction in mammals. 

 The special nerves presiding over these muscles (3, 4, 6) have been 

 pointed out already (p. 261). In the figure, the cut orljital ends of 

 them all are reflected away from the ball to disclose the underlying 

 muscles of the winker: the reader must mentally bring the six 

 loose ends together and fasten them to the bony orbit at points 

 near about opposite i, as above said of their origins. 



The above are the principal circumstances and accessories of the 

 optic apparatus ; we may now examine the eye itself, of which Fig. 

 82 gives an enlarged view, in longitudinal vertical section, — the 



Fig. 82. — Vertical antero-posterior section of eyeball : a, optic nerve ; b, sclerotic, its outer 

 coat ; c, sclerotic, its middle and inner coats ; d, choroid ; c, hyaloid ; /, marsupium ; ^, cornea ; 

 A, \ bony plates between sclerotic layers ; i, i, corrugations of choroid, forming ciliary pro- 

 cesses ; &, fc, canal of Petit ; I, I, iris ; m, anterior chamber of eye ; u, capsule of the lens ; 0, 

 lens ; j), posterior chamber of eye. Neither the retina, nor the peculiar sheathing of the optic 

 nerve, is shown. The nerve, marsupium, and ciliary processes, not falling in this section, can 

 only be arbitrarily shown. 



nerve, marsupium, and ciliary processes not indeed lying as shown 

 in this section, but so introduced as to display them intelligibly. 

 A bird's eyeball is not nearly so spherical or globular as a mam- 

 mal's. The globe of the human eye is about a five-sixths segment 

 of a large sphere (sclerotic) with a one-sixth segment of a smaller 

 sphere protruding in front (corneal). The anterior part of the 

 sclerotic of a bird is so prolonged as to be in some cases almost 

 tubular or cylindric, and the corneal protuberance is very convex : 

 the figure may be likened to an acorn which has a short blunt 

 kernel in a heavy shallow cup, or to a thick old-fashioned watch 

 with a very convex crystal. This characteristic shape is fairly shown 



