EYE OF THE ENGLISH SPARROW 411 



The fibers extend from the margin of the pupil to the circum- 

 ference of the iris. They he close to the pars retinalis iridis 

 (fig, 28, R). These are the dilator muscles of the iris. 



There is thus a marked difference in the character of the mus- 

 cles of the iris in birds and in man. Striated muscles are neces- 

 sary in the bird because of the rapid locomotion. In its flight 

 light conditions change in rapid succession. The amount of 

 light entering the eye, which is regulated by the size of the 

 pupil, must be regulated as rapidly as the conditions change. 

 This could not possibly be accomplished if the muscles of the 

 iris were composed of smooth muscle fibers as they are in man. 



Steinach ('90) has made a careful study of the reaction of the 

 iris in different vertebrates. He claims that the iris of the bird 

 reacts only to direct stimulation. That is, each is independently 

 stimulated, and a stimulation of one eye does not cause a reac- 

 tion of the iris in the other eye. He finds this true of those 

 vertebrates which have a complete decussation of the optic 

 nerve-fibers. I have not verified these facts in my study of the 

 sparrow, but I have demonstrated that the nictitating mem- 

 brane of each eye does not necessarily move simultaneously. 

 When both eyes are subjected to the same conditions the mem- 

 branes usually sweep across the eyes at the same time. Numer- 

 ous observations were made when one eye was exposed to bright 

 light and the other shaded. In each case the membrane moved 

 across the eye exposed to the light several times more per minute 

 than that of the protected eye. 



The ciliary muscles, or muscles of accommodation, in the 

 sparrow consist of striated fibers arranged in an anteroposterior 

 direction. . Their anterior attachment is to the inner lamella of 

 the sclera near its union with the cornea. Posteriorly, these 

 muscle fibers terminate in a tendon which extends backward 

 between the chorioid and sclerotic coats to the region of the ora 

 serrata where it is attached to the sclera. From the region of 

 the ciliary processes backward these fibers are rather firmly 

 united to both the chorioid and sclera. This is shown in figure 

 33, where distortion by the preserving fluids, has caused the 

 muscle and tendon fibers to be torn or pulled apart rather than 



