367 



formerly advanced by PETIT and afterwards urged by TREVIRANUS, 

 that ihepecten, by absorbing the rays of light that enter the eye 

 obliquely, limits the power of seeing objects that are out of the 

 direction of the axis of the ball, and renders vision in that direction 

 more definite and sharp. The ligamentum ciliare, which in the mam- 

 mals contains unstriped muscular fibres, here contains striped mus- 

 cular fibres; between its two laminae is an open space (canalis Fon- 

 tance) ; the iris possesses a high contractility, and presents muscu- 

 lar fibres which run round the pupil in concentric circular bundles, 

 and lie under the external layer of pigment upon which the diffe- 

 rent colour of the iris (commonly yellow or brownish) depends. 

 The pupil is usually round. The lens is commonly more convex 

 in proportion as the cornea is more flat, as in swimming birds; it is, 

 however, on the whole, much less convex than in fishes and reptiles, 

 and is placed far from the convex cornea, so that the anterior cham- 

 ber of the eye is spacious, and the quantity of the aqueous humour 

 considerable. The bird's eye is distinguished from that of mam- 

 mals not only by the pecten but also by the disposition and course 

 of the ciliary nerves; these in mammals penetrate the eye-ball and 

 iris on every side, in birds, on the contrary, only on one side of 

 the eye, the inferior, and afterwards divide into branches which are 

 distributed circularly over the iris 1 . 



Three eye-lids are present; besides the two horizontal eye-lids, 

 the upper and the lower, here there is always a third vertical eye- 

 lid at the inside, the membrana nictitans, which is transparent and 

 moderates the too powerful action of the light, by drawing itself 

 like a curtain over the anterior surface of the eye-ball. In different 

 birds, diurnal birds of prey for instance, in the toucan (Rhamphas- 

 tos), in Casuarius, in Struthio there is found a row of black eye- 

 lashes, or stiff, imperfect feathers along the margin of the horizontal 

 eye-lids. The lacrymal gland is commonly small, round, and 

 reddish, and is situated upwards at the posterior angle of the eye. 

 Birds have, in addition, a gland called Harder's gland (see above, 

 p. 232) on the anterior surface of the eye-ball, of which the excre- 

 tory duct opens behind the membrana nictitans. This membrane 



1 KIESER in OKEN'S und KIESEB'S Beitr. zur vergl. Zodogie, Anatomic u. Physlol. 

 1807, n - s. 98; see figures in TREVIRANUS Beitr. zur Anat. u. Physiol. der Sinnes- 

 werlczeuge. Bremen, 1828, fol. Heft i, Tab. I. f. 12, 13. 



