STRUCTURE OF THE EYE. 151 



aqueous, or watery, which is divided into two portions by the 

 iris ; the space from which to the cornea, f, is named the aii- 

 terior chamber of the aqueous humour, while that from the cor- 

 nea to the lens, d, is named its posterior chamber. Let us now 

 examine these parts a little more minutely. 



The Choroid Membrane, 6 &, is a filmy layer, which lies im- 

 mediately within the sclerotic, and extends as far forward as 

 the ciliarif circle, c. It is profusely covered with a substance 

 of the colour of China ink, named the pigment. 



Within the choroid coat is the Betina, which is the pulpy 

 expansion of the optic nerve, and the seat of the sensation of 

 sight. It extends over the greater part of the choroid coat, 

 but being extremely delicate, is apt to be in a great measure 

 destroyed in examining the parts. As already mentioned, the 

 optic nerve, g, in entering the eye becomes suddenly attenuated, 

 and presents itself internally of a linear form, in a fissure of 

 the membrane, h, from which the retina expands. 



At this point is a body or part peculiar to the class of birds, 

 varying in form and extent in the different species ; but in the 

 Buzzard, presenting the appearance of a delicate membrane, 

 nearly four-twelfths of an inch long, three-twelfths in height, 

 and composed of twenty plaits, disposed in the manner of a 

 frill or ruffle. On this membrane, once considered as muscular, 

 and named the onusculus pectinatiis, are ramified the branches 

 of the ophthalmic artery, which enter along with the optic 

 nerve. Its uses are not known. 



Opposite the posterior margin of the circle of sclerotic bones, 

 the choroid membrane divides into two laminre, of which the 

 inner becomes much thicker, and forms a broad zone of radi- 

 ating fibres or plicae, covered with black pigment, and of which 

 the central extremities adhere to the lens. This zone, c, is 

 named the Ciliary Circle. The outer layer of the choroid mem- 

 brane proceeds forward, and unites with the Iris, e, which is a 

 broad zone, composed of fibres, of which those of its outer part ra- 

 diate toward the pupil, while the inner, or those surrounding the 

 pupil, are circular. This membrane is extremely contractile and 

 dilatable. When the radiating fibres contract, the pupil is enlarg- 

 ed, and when the circular fibres contract, it is reduced in size. 



