58 THE MYOLOGY OF THE RAVEN. 



obliquely downward over the fore-part of the eyeball 

 The tendon of the pyramidalis gains the due direction 

 for that action by winding round the optic nerve, 

 and it is restrained from pressing upon the nerve by 

 the counteracting force of the quadratus, which thus 

 augments the power of the antagonist muscle, while it 

 obviates any inconvenience from pressure on the optic 

 nerve, which its peculiar disposition in relation to that 

 part would otherwise occasion. The nictitating mem- 

 brane returns on the relaxation of its muscles, by virtue 

 of its own elasticity, to the inner corner of the orbit, 

 where it lies folded when not in use " (Anat. of Verts., 

 vol. ii. p. 143). 



42. Obliquus superior. As in the Mammalia, the 

 movements of the eyeball depend upon the action of two 

 oblique muscles and four recti muscles (Fig. 22). 



The oblique muscles arise close together at the inner 

 and superior angle of the pars plana, their exact origins 

 being shown in the figure. Between them passes the 

 olfactory nerve. The origin of the superior oblique is 

 situated the more posteriorly, and rather lower than 

 that of the obliquus inferior. Its diverging fibres pass 

 backwards and outwards, forming a pretty, fan-shaped 

 little muscle, to spread over the upper side of the eye- 

 ball, their extremities being overlapped by the rectus 

 superior. In none of these eye-muscles proper do the 

 terminations of the fibres of insertion reach so far as the 

 osseous circlet formed of the sclerotal plates ; and it 

 is evident, further, that, owing to the greatly confined 

 condition of the eyeball, the necessity for a ligamentous 

 pulley to assist the action of the present muscle is 

 obviated. 



43. The obliquus inferior muscle arises, as we have 

 just stated, close to the last described one (Figs. 20, 22, 



