148 PRACTICAL ORNITHOLOGY. 



it over the eye eftectually and expeditiously, so as not to obstruct 

 vision for an instant, the tendon attached to its free or poste- 

 rior edge might be placed high toward the outer edge of the 

 orbit ; but when the membrane was retracted, the tendon would 

 be across the eye, or would require some mechanism inconsis- 

 tent with the free use of these parts. The tendon is therefore 

 carried over the edge of the eyeball, and as it must be drawn 

 upwards, it is attached to a muscle, the quadratus, placed high 

 on the eyeball ; but as this direct connexion would leave the 

 tendon too short, it is continued farther, curves downward, and 

 ends in another muscle placed near the lower and anterior edge 

 of the eyeball. The uses of the membrana nictitans are to 

 clear the eye of extraneous objects, as dust, accidentally intro- 

 duced, to moisten its surface by diffusing the lachrymal fluid 

 over it, and occasionally to protect it from the light. 



Having removed from the orbit, or cavity in which it was 

 contained, the Eyeball, we observe that it is of enormous size 

 in proportion to that of a quadruped, being an inch and one 

 eighth in diameter. Its form is not globular, but composed of 

 two segments of unequal spheres, and an intermediate portion. 

 Viewed anteriorly. Fig, 5, it presents the transparent con- 

 vex cornea, the iris and pupil ; a dark coloured rim to which the 

 cornea is attached, like a watch-glass in its case, then a circle of 

 bony plates, included in the fore part of the rough glistening 

 membrane, or sclerotica, which bounds the posterior part of 

 the eye. Viewed from behind, Fig. 6, it presents a small 

 segment of a large sphere formed by the sclerotic coat, the optic 

 nerve, cut across at its entrance into the eye, and the muscles 

 by which the eyeball is moved. Of these there are six, four 

 straight, and two oblique. The recti, or straight muscles, 

 arise from the bottom of the orbit, around the aperture for 

 the passage of the optic nerve, and are attached to the eye by 

 thin tendons, of which the fibres blend with those of the scle- 

 rotic coat. The rectus superior, marked c, by pulling down the 

 upper edge of the eyeball, directs the axis of the eye upwards, 

 and is thus named attollens oculi. The rectus inferior, or depri- 

 meus, d, has the opposite eifect. The rectus posticus, exter- 

 ntis, or ahducens, e, directs the axis of the eye outwards, while 



