EVE 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 
with 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 portions. 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 different 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 pecten, 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 Dromeus, 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 Reptiles, is not 
the screening off of light, but the vascular supply or alimentation of 
the vitreous humor. 
The lens is a biconvex absolutely colourless 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 
