306 



AVES. 



light. It is inclosed, as in Mammalia, in a 

 distinct capsule, which adheres very firmly to 

 the depression in the anterior part of the 

 vitreous humour; the capsule is itself lodged 

 between two layers of the membrana hyaloidea, 

 which, as they recede from each other to pass 

 the one in front and the other behind the lens, 

 leave round its circumference the sacculated 

 canal of Petit. 



The vessels of the lens are derived from 

 those of the marsupium, which, as we have 

 before observed, are ramifications of the analogue 

 of the arteria centralis retinae. With respect to 

 this vessel we may here observe, that it is not 

 continued as a simple branch from its origin to 

 the marsupium, such a course would be in- 

 consistent with the important functions it is 

 destined to fulfil in the present Class. Imme- 

 diately before penetrating the coats of the eye 

 it breaks into numerous subdivisions, the aggre- 

 gate of which is much greater than the trunk 

 whence they proceed, and these again unite, 

 forming a plexus (e,fg. 139) close to the ex- 

 ternal side of the optic nerve. The artery of 

 the marsupium proceeds from this plexus, and 

 runs along the base of the folds, giving off at 

 right angles a branch to each fold, which in 

 like manner sends off smaller ramuli. The 

 plexus at the origin of the marsupial artery 

 serves as a reservoir for supplying the blood 

 required for the occasional full injection of the 

 marsupium ; and a similar but larger plexus 

 (4, Jig. 139) is formed at the origins of the 

 ciliary arteries which supply the erectile tissue 

 of the ciliary processes and iris. These plexuses 

 are described by Barkow, from whose Memoir* 

 the subjoined figure is taken, but their relation 

 to the erectile powers of the parts they supply 

 appears to have escaped his notice. 



The vitreous humour presents few peculia- 

 rities worthy of note ; compared with the aque- 

 ous humour, it is proportionally less in quan- 

 tity than in the eyes of Mammalia. The outer 

 capsule formed by the hyaloid membrane is 

 stronger, and can 

 from the humour. 



The Eye-ball 

 is moved in Birds 

 by four straight 

 and two oblique 

 muscles. The 

 Recti muscles a- 

 rise from the cir- 

 cumference of the 

 optic foramen and 

 expand, as they 

 pass forward, to 

 be inserted into 

 the soft middle 

 part of the scle- Muscles of the eye. 



rotic. We have not been able to trace their 

 insertion distinctly to the osseous circle ; 

 their aponeurosis cannot be reflected for- 

 wards from the sclerotica without lacerating 



o 



that membrane. 



The Obliqui both arise very near together 

 from the anterior parietes of the orbit, and go 



* Meckel's Archiven, B. xii, pi. x. 



v 



be more easily separated 

 Fig. 139. 



to be inserted, the one into the upper, the other 

 into the lower part of the globe of the eye ; the 

 superior obliquus does not pass through a, 

 pulley, as in Mammalia. 



All the muscles are proportionally short in 

 this class, but especially so in the Owls, in 

 which the eye, from its large size and close 

 adaptation to the orbit, can enjoy but very little 

 motion. 



In the subjoined figure and in fig. 140, 

 a' is the rectus superior or attollens ; b' the 

 rectus inferior or deprim&u ; c' the rectus ex- 

 ternus or abducens ; d' the rectus interims or 

 adducens ; e 1 the obliquus superior; f the ob- 

 liquus inferior ; g' the quadratics ; h' the pura- 

 midalis. 



The accessory parts of the eye in Birds are 

 similar to those of the higher Reptiles. There 

 are three eye-lids, two of which move vertically, 

 and have a horizontal commissure, while the 

 third, which is deeper-seated, sweeps over the 

 eye-ball horizontally, from the inner to the 

 outer side of the globe. The vertical, or upper 

 and lower eye-lids, are composed of the com- 

 mon integument, of a layer of conjunctiva, and 

 between these of a ligamentous aponeurosis, 

 which is continued into the orbit, and lines the 

 whole of that cavity. The lower eye-lid is the 

 one which generally moves in closing the eye in 

 sleep, and it is further strengthened by means 

 of a smooth oval cartilaginous plate, which is 

 situated between the ligamentous and con- 

 junctive layers. 



The orbicularis muscle is so disposed as by 

 means of this plate to act more powerfully in 

 raising the lower than in depressing the upper 

 eye-lid. In the latter it is continued imme- 

 diately along the margin : in the lower eye-lid 

 the tarsal cartilage intervenes between the mus- 

 cle and the ciliary margin. 



The levator palpe.bra superioris arises from 

 the roof of the orbit, and is inserted near the 

 external angle of the lid. 



There is also an express muscle for depress- 

 ing the lower eye-lid, as in the Crocodile. 



In the Owls and Night-jar (Caprimu/gus) 

 the eye-lids are closed principally by the depres- 

 sion of the upper one. There are but few birds 

 that possess eye-lashes ; of these the Ostrich is 

 an example, as also the Horn-bills and the Owls, 

 in which they are arranged in a double series ; 

 but in these they are rather to be considered as 

 feathers with short barbs, than true eye-lashes. 



The third eye-lid, or membrana nictitans, is 

 a thin membrane, transparent in some birds, 

 in others of a pearly white colour, which, 

 when not in action, lies folded back by virtue 

 of its own elasticity on the inner or nasal side 

 of the globe of the eye, with which it is in close 

 contact. 



Two muscles are especially provided to effect 

 its movements, but are so placed as to cause 

 no obstruction to the admission of light to the 

 eye during their actions. One of these is 

 called the Quadratics nictitantis, (g,ftg- 139;) 

 it arises from the sclerotica at the upper and 

 back part of the globe of the eye, and its fibres 

 slightly converge as they descend towards the 

 optic nerve, above which they terminate in a 



