138 ANATOMY OF VERTEBRATES. 



seem in some Birds to be voluntary. The contraction and dilata- 

 tion of the pupil, independently of any change in the quantity of 

 light to which the eye is exposed, is most conspicuous and re- 

 markable in the Parrot tribe, but it has been observed also in the 

 Cassowary and other birds. ^ 



The colour of the iris is subject to many varieties, Avhich fre- 

 quently display great brilliancy, and afford zoologists distinguish- 

 ing specific characters of Birds ; although these cannot always be 

 implicitly relied upon. The breadth of the iris varies in diffe- 

 rent species, but is greatest in Birds which take their food in the 

 gloom, e. g.. Owls and Nightjars, in order that the pupil may be 

 proportionally enlarged to admit as much light as possible to the 

 retina. The ciliary nerves and vessels run in the form of single 

 trunks between the choroid and sclerotica, and terminate ante- 

 riorly 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 plicae radiating towards the 

 anterior part of the crystalline lens, where they terminate in 

 slightly projecting ciliary pj^ocesses, fig. 57, d, the extremities of 

 which adhere firmly to the capsule of the crystalline. 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 marsuplum 

 or pecten, ib. f, which is a plicated vascular membrane analogous 

 in strvicture to the choroid, and equally blackened by the pig- 

 mentum ; situated in the vitreous humovir anterior to the retina, 

 and extending from the point where the optic nerve penetrates 

 the eye to a greater or less distance forward, being in many Birds 

 attached to the posterior j^art of the capsule of the lens. 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, ib. g, 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 mem- 

 brane, and directed downward and obliquely forward. The 

 central or inner layer of this sheath is split longitudinally, and the 

 plicated substance of the nerve, fig. 48, 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 



• vir. p. 304. 



