142 



ANATOMY OF VERTEBKATES. 



59 



Muscles of the eye. Goose. 



compared with the aqueous humour, it is proportionally less in 

 quantity than in the eyes of Mammals. The outer capsule formed 

 by the hyaloid membrane is stronger, and can be more easily 

 separated from the humour. 



The eyeball is moved in Birds by four straight and two ob- 

 lique muscles. The Recti muscles arise from the circumference 

 of the optic foramen, and expand, as they pass forward, to be 

 inserted into the soft middle part of the sclerotic. We have not 



been able to trace their insertion dis- 

 tinctly to the osseous circle ; their apo- 

 neurosis cannot be reflected forward from 

 the sclerotica without lacerating that 

 membrane. 



The Obliqui both arise very near to- 

 gether from the anterior parietes of the 

 orbit, and go to be inserted, the one into 

 the upper, the other into the lower part 

 of the globe of the eye ; the superior 

 obliquus does not pass through a pulley, 

 as in Mammalia. All the muscles are 

 proportionally short in this class, but especially so in the Owls, 

 in which the eye, from its large size and close adaptation to the 

 orbit, can enjoy but very little motion. In figs. dQ and 59, a is 

 the rectus superior or attollens ; h the rectus inferior or deprimens ; 

 c the rectus externus or abducens ; d the rectus internus or addu- 

 cens \ e the obliquus superior-, /the obliquus inferior-, g the 

 quadratus ; h the pyrainidalis. 



The accessory parts of the eye in Birds are similar to those of 

 the higher Reptiles. There are three eyelids, two of which move 

 vertically, and have a horizontal commissure, while the third, 

 which is deeper-seated, sweeps over the eyeball horizontally, 

 from the inner to the outer side of the globe. The vertical, or 

 upper and lower eyelids, are composed of the common integu- 

 ment, of a layer of conjunctiva, and between these of a liga- 

 mentous aponeurosis, which is continued into the orbit, and lines 

 the whole of that cavity. The lower eyelid is the one which 

 generally moves in closing the eye in sleep, and it is further 

 strengthened by means of a smooth oval cartilaginous plate, which 

 is situated between the ligamentous and conjunctive layers. 



The orbicularis muscle is so disposed as by means of this plate 

 to act more powerfully in raising the lower than in depressing 

 the upper eyelid. In the latter it is continued immediately along 



