EYE 231 



anterior sclerotic ring ; it is thrown into numerous radiating folds, 

 and immediately behind the base of the iris surrounds the margin 

 of the lens, which it connects through the ligamentum pectinatum 

 Avith the anterior margin of the sclerotic ring, and thus holds the 

 lens in position. 



The ciliary muscle is of importance for the accommodation of the 

 eye to varying distances. It consists of numerous striated or volun- 

 tary muscular fibres, which are situated partly within the corpus 

 ciliare. The whole muscle consists of several j)ortions. The 

 anterior one, " Crampton's muscle," arises from the anterior margin 

 of the sclerotic ring and is inserted upon a small circular ridge of the 

 cornea. The chief portion, "Miiller's muscle," extends from the same 

 ridge backwards into the chorioid ; other fibres likewise arising from 

 this ridge pass into the ligamentum pectinatum. The mechanism of 

 the accommodation of the eye is very complicated ; it amounts to 

 this that it can be most rapidly adjusted to difi"erent distances, not 

 through a change in the convexity of the cornea or through a forward 

 or backward motion of the lens, but through a change in the con- 

 vexity of the lens itself. 



The jjecten, comb or fan of the eye, is a peculiar lamella of 

 the chorioid which projects from the entrance of the optic nerve 

 far into the posterior chamber of the eye. It is present, so far 

 as known, in all birds except Apteryx, and is a wedge -like or 

 rhomboid, deeply-pigmented black lamella, which is thrown into a 

 variable number of folds. The number of these folds (3 in Capri- 

 mulgus, 4 in Dromseus, 14 to 20 in Struthio, 20-30 in Crows) varies 

 in closely-allied birds, and is of no systematic value. The use of 

 this organ, which is absent in Mammals and most Eeptiles, is not 

 the screening off of light, but the vascular supply or alimentation of 

 the vitreous humor. 



The lens is a biconvex absolutely coloiu'less and transparent body 

 of considerable refractory power. Its broad diameter amounts to 

 about 1"3 of its axis; the anterior surface is more convex than 

 the posterior ; the lens is composed of numerous mostly concentric- 

 ally-arranged layers. 



The lens, being held in position by the ligamentum pectinatum 

 of the corpus ciliare, divides the whole of the inner space of the 

 eye into an anterior and a posterior chamber. The anterior cham- 

 ber is filled with the colourless aqueous humor, while the vitreous 

 humor, which fills the posterior larger chamber, is of a more 

 gelatinous but likewise colourless consistency. 



The retina is a thin expansion of the optic nerve over the inner 

 surface of the chorioid membrane, and extends over the posterior 

 or basal portion of the eye, ending at the beginning of the ciliary 

 body. The retinal membrane is scarcely 0"3 of a millimetre in 

 thickness, and is, as continuation of the optic nerve, composed of 



