304 



AVES. 



together with a tough ligamentous substance, 

 which seems to be the continuation of the scle- 

 rotic between the edges that overlap each other. 



The cornea possesses the same structure as 

 in mammalia, but differs with respect to form. 

 When the posterior part of the eye is com- 

 pressed by the muscles, the humours are urged 

 forwards and distend the cornea ; which, at that 

 time, becomes much more prominent in most 

 birds than it is ever observed in mammalia ; 

 and under such circumstances, the eye is in a 

 state for perceiving near objects. When the 

 muscles are quite relaxed, the contents of the 

 eye-ball retire to the posterior part, and the 

 cornea becomes flat or even depressed : this is 

 the condition in which we always find the eye 

 of a dead bird, but we can have no opportunity 

 of perceiving it during life. It is only prac- 

 tised for the purpose of rendering objects visi- 

 ble that are placed at an extreme distance. 

 From the well-known effects of form upon re- 

 fracting media, it must be presumed, that the 

 cornea possesses very little, if any, convexity, 

 when a bird which is soaring in the higher re- 

 gions of the air, and invisible to us, discerns its 

 prey upon the earth, and descends with uner- 

 ring flight to the spot, as is customary with 

 many of the rapacious tribe. 



The degree of convexity of the cornea is also 

 changed in birds by the action of muscular 

 fibres especially appropriated to its motions. 

 These were discovered by Crampton ; are dis- 

 posed around the circumference of the cornea, 

 and are attached to its internal layer; they 

 draw back the cornea, in a manner analogous 

 to the action of the muscles of the diaphragm 

 upon its tendinous centre. 



The choroid coat re- 



Fig. 138. 



sembles in its structure 

 that of mammalia; it 

 is copiously covered 

 with a black pigment, 

 similar to that in the 

 human eye. Opposite 

 the bony circle the 

 choroid separates into 

 two layers; the exter- 

 nal layer is the thin- 

 nest, and adheres at first firmly to the sclerotica, 

 after which it is produced freely inwards to 

 form, or be continuous with, the iris. 



The iris (e,fg. 138) is delicate in its texture, 

 which under the lens appears composed of a fine 

 net-work of interlacing fibres,but it is remarkable 

 for the activity and extent of its movements, 

 which seem in many birds to be voluntary. The 

 contraction and dilatation of the pupil, inde- 

 pendent of any change in the quantity of light 

 to which the eye is exposed, is most conspicu- 

 ous and remarkable in the Parrot tribe, but we 

 have observed it also in the Cassowary and 

 some other birds. 



The colour of the iris is subject to many 

 varieties, which frequently display great bril- 

 liancy, and afford zoologists distinguishing spe- 

 cific characters of birds ; although these cannot 

 always be implicitly relied upon. 



The breadth of the iris varies in different 

 species, but is greatest in Birds which take 



their food in the gloom of evening, as the 

 Owls and Night-jar, in order that me pupil 

 may be proportionally enlarged to admit as 

 much light as possible to the retina. Carus 

 observes that in the eye of the Owl is exhibited 

 with peculiar distinctness the remarkable dis- 

 tribution of the ciliary nerves and vessels, which, 

 rnnning in the form of single trunks between 

 the choroid and sclerotica, terminate anteriorly in 

 several ring-shaped plexuses for the supply of 

 the iris and of the muscular circle of the cornea. 

 The pupil is usually round : in the Goose and 

 Dove it is elongated transversely, and in the 

 Owls is vertically oval. 



The inner layer of the choroid is thicker than 

 the external, and is disposed in numerous 

 thickly set plica radiating towards the anterior 

 part of the ehrystalline lens, where they termi- 

 nate in slightly projecting ciliary processes, (d, 

 tfg.!38,)the extremities of which adhere firmly to 

 the capsule of the ehrystalline. These processes 

 are the most numerous, close set, and delicate in 

 the Owl ; they are proportionally larger and 

 looser in the Ostrich. 



The chief peculiarity in the eye of the Bird 

 is the marsupium or pec ten, (J\Jig. 138,) which 

 is a plicated vascular membrane analogous in 

 structure to the choroid, and equally blackened 

 by the pigmentum; situated in the vitreous 

 humour anterior to the retina, and extending 

 from the point where the optic nerve penetrates 

 the eye to a greater or less distance forwards, 

 being in many birds attached to the posterior 

 part of the capsule of the ehrystalline. As its 

 posterior point of attachment is not to the 

 choroid but to the termination of the optic 

 nerve, this requires to be first described. 



When the optic nerve arrives at the sclerotic, 

 it tapers into a long conical extremity, which 

 glides into a sheath of a corresponding figure, 

 excavated in the substance of that membrane, 

 and directed downwards and obliquely forwards. 

 The central or inner layer of this sheath is split 

 longitudinally, and the substance of the nerves 

 passes through this fissure. A similar but 

 longer fissure exists in the corresponding part 

 of the choroid : so that the extremity of the 

 optic nerve presents in the interior of the eye, 

 instead of a round disc, as in mammalia, a 

 white narrow streak, from the extremities and 

 sides of which the retina is continued. Branches 

 of the ophthalmic artery, which are quite dis- 

 tinct from the vessels of the choroid, and ana- 

 logous to the arteria centralis retina, enter the 

 eye between the laminse of the retina, along the 

 whole extent of the oblique slit above men- 

 tioned, and immediately enter or compose the 

 folds of the marsupial membrane, upon which 

 they form most delicate and beautiful arbore- 

 scent ramifications. 



The marsupium is lodged like a wedge in 

 the substance of the vitreous humour, in a 

 vertical plane, directed obliquely forwards. In 

 those species in which the marsupium is widest, 

 the angle next the cornea reaches the inferior 

 edge of the capsule of the ehrystalline; but 

 where it is narrow, the whole anterior surface 

 is in contact with the same point. This con- 

 tact is so close in some birds, as the Vulture, 



