)0 



INTRODUCTION. 



plaited membrane, named the pecten, projecting from the retina 

 in the direction of the chrystaUine lens. The pnpil is, I be- 

 lieve, always round, although it has been alleged to be trans- 

 versely or longitudinally elliptical in certain species, the Owls 

 and Pigeons for exam23le ; the iris extremely contractile, and 

 frequently of the most vivid colours, although for the most part 

 of different shades of brown. The eyeball is moved by six 

 muscles, four of which are straight and two oblique ; but in 

 many birds it possesses very little motion, and in some of the 

 Owls is so closely fitted into the orbit as to be absolutely im- 

 moveable. There are two eyelids, of vfliich the lower is gene- 

 rally largest, and a nictitant membrane moved by two muscles 

 so as to sweep over the surface of the cornea. 



^-^^^ ...r,*^ ^ig- 1^- represents the eyeball of an 



Owl viewed laterally : — «, the cornea, 

 which is very convex ; Z>, the sclerotica, 

 depressed, and at its anterior part, c, 

 contracted and surrounded by bony 

 plates. Fig. 11. is an anterior view of the same, in which the 

 central dark part represents the pupil dilated, the zone sur- 

 rounding it the iris, then the circle of bony plates ; and exter- 

 nally the sclerotic coat. 



Fig. 12. represents the head of a Thrush, Turdus musicus : 

 «, the upper mandible ; &, the lower mandible ; c, the tongue ; 

 e, the nostril ; ^, the feathers covering the ear. Fig. 13. is 

 the external ear of the same bird, margined with feathers, of 

 some of which only the shafts are represented. Fig. 14. is the 

 roof of the mouth. Fig. 15. the tongue, a ; and the glottis or 

 top of the windpipe, h. 



Fig. 11. 



Fig. 12. Head of a ThrusTi. 



Fig. 13. 



The organ of Ii earing has a somewhat less complex structure 

 than in the mammifera. There is no conch or external auricle 



