368 CLASS xvr. 



is moved by a muscle (musculus pyramidalis) which arises on the 

 posterior surface of the sclerotic under the entrance of the optic 

 nerve and has a very long tendon which passes above the optic 

 nerve, and is inclosed in the canal of a second- broad muscle (muse, 

 quadratus) on the upper and back part of the sclerotic ; on leaving 

 this canal the tendon proceeds over the eye-ball forward and down- 

 ward to be inserted into the margin of the third eye-lid 1 . The 

 eye-ball has also its six proper muscles, as in man, four straight 

 and two oblique; they partly cover the two preceding muscles, and 

 are short: the ball has less mobility in birds, and in them the direc- 

 tion of their sight to different objects is effected less by the motions 

 of the eye than of the whole head 2 . 



The organ of hearing presents in birds an adherence to a 

 general form in a much greater degree than in reptiles, where in 

 different orders it stops at very different grades of development. In 

 addition to the vestibule the auditory organ has in all birds a tym- 

 panic cavity, an auditory ossicle, a tympanic membrane, an Eusta- 

 chian tube, and an external auditory passage. A thin bony lamina 

 surrounds the membranous vestibule, and, since this lamina is en- 

 veloped by a cellular and loose bony tissue, the bony vestibule in 

 birds may be easily exposed. The three semicircular canals are 

 large. The anterior and inner is the largest ; it runs from before 

 backwards with its curvature upwards, and unites by its posterior 

 extremity with that of the posterior canal, whose arch is turned 

 outwards and stands vertically ; the third canal lies almost at a right 

 angle under and across the direction of these, and thus is horizon- 

 tal; it has two openings into the vestibule. The vestibule is small. 

 The cochlea has the form of an obtuse cone, which at its extremity 

 swells into an oval tubercle. The interior of this non-convoluted 

 cochlea is divided by a thin membrane into two cavities; the mem- 

 brane is stretched between two bent cartilaginous strips connected 



1 Here, for the purpose of keeping the tendon in its place, a small bony swelling is 

 sometimes found on the bony eye-ring. Compare on these muscles NITZSCH Osteogr. , 

 Beitr. s. 80, 8 1, Taf. I. figs. 6, 7; see also the drawings and description of these 

 muscles in the eye of the ostrich, in Catalogue of tlie Physiol. Series in the Museum of 

 the College of Surgeons, in. pp. 206 208, PI. 42, figs. 4 7. 



3 Compare on the eye of birds KIESER Ueber die Metamorphose des Auges dcs 

 lebriiteten ffiihnchens, 1. 1. s. 89 108, W. D. SCEMMERRING De oculor. sectione hori- 

 zontali, pp. 47 55, &c. 



