THE MUSCLES OF THE EYE AND EAR. 59 



oh. inf.). Its fibres pass in exactly the same direction, 

 differing only in their downward, rather than the upward 

 inclination, in order to become inserted in the sclerotic 

 of the inferior aspect of the eyeball, where they are 

 overlap])ed by the inferior rectus muscle. 



The action of these two oblique muscles is too simple 

 to require a detailed description here. They are the 

 direct antagonists to each other, and are principally con- 

 cerned in movements of torsion of the eye. 



44. Rectus superior. — The scA^eral origins of the recti 

 muscles circumscribe the optic foramen, just wdtliin its 

 free border. The rectus superior and internus surround 

 its upper half, while the rectus inferior and externus 

 arise below. The rectus externus arises by two distinct 

 heads, between which the sixth nerve passes out of 

 the brain-case through a small circular foramen which 

 appears to be constant in the Eaven (Fig. 22). 



The fibres of the rectus superior diverge as they pass 

 forwards, upwards, and outwards, the innermost ones 

 overlapping those of the obliquus superior at right 

 angles. They are inserted into the sclerotic just within 

 the margin of the bony circle of the eye (Fig. 20). 



45. The rectus inferior muscle is to the rectus superior 

 exactly what the oblicpius inferior muscle is to the 

 obliquus superior. Its fibres spreading out like a fan, as 

 they pass from their origin, are inserted on the lower 

 aspect of the eyeball, in the same manner as the fibres 

 of the rectus superior are inserted upon its upper side. 



46. The rectus externus seems to be the shortest of all 

 the recti muscles ; and, as I have already remarked, it 

 arises by tw^o heads, disposed as shown in Fig. 22. It 

 also is fan-shaped, and finds insertion, in a w^ay similar 

 to the two foregoing muscles, on the external, or what 

 is really the posterior side, of a Eaven's eye. 



