646 BIRDS 



a repeated forward movement, and detachment, of the retina during 

 'wood-pecking'. The chorioid in most birds is highly ductile in the direc- 

 tion of its thickness, i. e. radially of the globe : Abelsdorff and Wessely 

 found that if the anterior chamber of a bird is drained by corneal punc- 

 tures, the chorioid will promptly thicken enormously through engorge- 

 ment. 



The avian chorioid is not known ever to contain a tapetum lucidum, 

 though some old accounts, not since substantiated, mention one for 

 certain owls. The eyeshine of the goatsuckers (nighthawks, whip-poor- 

 wills, etc.) is so vivid however that the eyes of these birds, for this and 

 other reasons, seem most attractive objects for study. 



Anteriorly, the chorioid thins out and becomes the base-plate of the 

 ciliary body (Fig. 191), which angles sharply inward toward the axis 

 and leaves a large space between itself and the sclera, to be traversed by 

 the many strands of the pectinate ligament. The ciliary processes occupy 

 the whole ciliary zone (so that there is no true orbiculus) , and are very 

 numerous — sometimes numbering in the hundreds, though only a major- 

 ity are tall enough to reach the lens capsule and fuse therewith. The 

 number of processes goes roughly with the size of the eye, and from eye 

 to eye they do not vary greatly in thickness. From the ciliary processes, 

 and from between them, there originate the fibrils of the zonule, the 

 anteriormost of which are squeezed between the iris and the ringwulst, 

 as in lizards. 



The ciliary muscles, as in lizards, are more closely associated with the 

 sclera than with the uvea. They may be arranged as in Figure 191 

 or, probably much more commonly, the muscle of Briicke originates 

 from the inner side of the thin scleral sheet which forms the anchorage 

 of the pectinate ligament and covers the inner side of Crampton's muscle. 

 Briicke's muscle is sometimes divided into an anterior and a posterior 

 portion; then, the anterior is properly known as 'Miiller's muscle' — first 

 described by Miiller in the goshawk, Accipiter gen tills . Other variations 

 are mentioned on pp. 279-81 and 439-42. 



The 'canal of Schlemm' is complex, represented not by a single venous 

 annulus, but by two, with moreover an associated artery which lies be- 

 tween them (in Passer domesticus, two arteries), and has a likewise 

 annular course. The connections and relationship of the veins and 

 artery (s) are not yet known. The whole complex lies near the limbus, 

 attached by connective tissue to the inner surface of the sclera near the 

 anterior end of Crampton's muscle. 



