ORGANS OP THE SENSES. 269 



cornea is here removed to a distance from the iris by 

 the abundant aqueous humour occupying the anterior chamber, 

 and from the rapid evaporation of this fluid it quickly sinks to 

 flatness or to a concavity externally after death* The cornea is 

 least convex, and the eyes are smallest in the aquatic birds, 

 where the food is often distinguished more by the sen- 

 sibility of the lips than by the organs of vision, and they 

 are largest in the nocturnal birds of prey where they are 

 directed forwards, with their axes nearly parallel, and the 

 pupils are of great size, to receive the strongest impression 

 from a feeble light. The choroid coat, consisting as usual 

 of two layers, of a dark colour, is lined internally with 

 a thick deposit of black pigment which appears to be com- 

 posed of globular particles with a transparent centre, and 

 a prolongation of this dark membrane covered with its 

 pigment, is extended forwards through the vitreous humour, 

 from the lineal entrance of the optic nerve to the capsule 

 of the crystalline lens to which its anterior margin is attached. 

 This folded marsupium or pecten, continued from the 

 vascular layer of the choroid through the axis of the eye 

 to the lens, may convey the central nutritious vessels 

 of these humours, or moderate the intensity of the light 

 admitted into the eye, or may assist in adapting the eye 

 to different distances by affecting the position or the form 

 of the lens to which it is attached. From the ciliary liga- 

 ment which unites firmly the choroid to the sclerotic, the 

 large ciliary processes extend freely inwards to form a corona 

 around the margin of the lens ; the ciliary nerves unite 

 to form plexuses at this ciliary ligament, and the retinal 

 expansion of the optic nerve appears to terminate internally 

 at the same place. Plexuses are formed on the ciliary 

 arteries, as in similar delicate organs, before penetrating 

 the delicate textures of the eye to spread on the ciliary 

 processes, the marsupium, and the iris which is here remark- 

 able for the diversities of lively colours which it presents 

 in the different species. From the great mobility of the 

 iris, the round pupil of birds is susceptible of great and 

 rapid changes of dimension, which are very extensive and 

 almost voluntary in the parrots. The optic nerve penetrates 

 the choroid by a lengthened fissure, as in many of the lower 

 animals, and from this lineal perforation the falciform qua- 

 drangular pecten extends forwards through the thin and 



