EYE 229 



will doubtless find that artificial feathers can be made, even as 

 artificial flowers now are, and there will be a fine opening for the 

 ingenious inventor. The pity is that he does not at once begin. 



EYE. The eyeball of Birds is far less globular than that of 

 Mammals, resembling rather the tube of a short and thick opera- 

 glass. It consists externally of three successive portions. A basal 

 or posterior, an intermediate, and an anterior portion. The wall of 

 the anterior portion is formed by the transparent cornea, and is more 

 strongly curved than that of the basal portion, which like the inter- 

 mediate portion is formed by the white and opaque sclera. Within 

 the walls of the sclera exists cartilage with occasional ossifications. 

 Such an ossification, the posterior sclerotic ring, surrounds in many 

 birds, especially in the Pici and in the Passeres, the entrance of the 

 optic nerve. Nearly all birds possess an anterior sclerotic ring which 

 is composed of from 10 to 17, generally from 13 to 15, bony scales 

 \vhich overlap each other in various ways, and form the somewhat 

 conical intermediate or connecting portion of the walls of the eye. 



The outer surface of the cornea is covered by the likewise quite 

 transparent conjunctiva, a continuation of the mucous membrane 

 lining the inside of the eyelids. The inner surface of the cornea 

 is covered by the "membrana Descemeti," a structureless film 

 Avhich seems to be the continuation of the chorioid membrane. 



The inner surface of the sclerotic wall is covered by the chorioid 

 membrane, a thin membrane, which is rich in blood-vessels and is 

 dark or black owing to the number of pigment-cells. It is morpho- 

 logically the continuation of the pia mater or innermost sheath of 

 the optic nerve, which enters the middle of the posterior segment 

 of the eye and then spreads itself out as the retina upon the inner 

 surface of the chorioid membrane. The latter is consequently 

 situated between the sclera and the retina. Level with the 

 junction of the cornea and the sclera, i.e. at the anterior margin of 

 the intermediate portion of the eye, the chorioid membrane leaves 

 the wall of the eye by turning away at a right angle and hanging 

 like a circular curtain, the iris, over the anterior surface of the lens, 

 into the anterior chamber of the eye. The central hole in this 

 diaphragma-like curtain is the pupil. The iris is a thin plate of 

 connective tissue ; its hinder surface is covered like the chorioid 

 with a layer of black pigment, while its anterior side is coloured in 

 various ways, either by pigment corpuscles or by coloured drops of 

 fat. Often beautifully bright, it adds much to the expression of 

 birds ; it is, for instance, vividly yellow in Lamprocolius, Botaurus, 

 and Picus martins ; red in Chrysotis and in Nycticorax ; green in ,//'.' 

 the Cormorant; white in the Grey Parrot and in Hareld'; grey in v ^ 



Balearica pavonina and in Fratercula ; bluish in Cypselus ; black in 

 Cacatua, and so on. In most young birds the colour of the iris is, 



