THE EYE 425 



the other eyelids, between them and the eyeball. It 

 can be pulled obliquely downwards and backwards 

 over the front of the eyeball with great rapidity, 



2. The muscles of the eyeball. 



Open the orbit by removing its dorsal wall with strong 

 scissors, taking care not to damage the nictitating membrane. 



a. The muscles moving the eyeball are essentially the 



same as those of the skate or rabbit. 

 Remove the eyeball completely, cutting through the muscles 

 and the optic nerve as far as possible from the eyeball. 



b. The muscles of the nictitating membrane are two 



in number, and lie close to the inner or orbital 



surface of the eyeball. 

 Snip away with scissors the recti and oblique muscles. 



i. The quadratus, or bursalis muscle, is an oblong 

 sheet of muscle, lying in close contact with the 

 inner surface of the eyeball. Its fibres arise 

 along the dorsal edge of the eyebaU and 

 run downwards, ending just above the optic 

 nerve in a free border, which forms a 

 tubular tendinous sheath for the tendon of 

 the pyramidalis muscle, 

 ii. The pyramidali? is a small muscle, somewhat 

 triangular in shape. It arises from the inner 

 surface of the ventral part of the eyeball, 

 below, and a little anterior to, the entrance of 

 the optic nerve. From this origin its fibres 

 run upwards and backwards, converging to 

 form a long thread-like tendon, which, passing 

 through the tubular sheath of the quadratus, 

 runs down on the inner surface of the eye- 

 ball, and then under it, to be inserted into 

 the ventral border of the nictitating mem- 

 brane. 



By the combined action of these two 

 muscles the rapid movement of the nicti- 

 tating membrane is effected. 



