162 



by two muscles, the quadratus, which forms a loop or pulley, 

 through which the long tendon oi the pyramidal passes in its 

 course to the membrane; by the joint action of these two 

 muscles this curtain is drawn with great velocity over the 

 cornea, while its elasticity again restores or retracts it to 

 the inner canthus, the moment the muscles and the tendon 

 are relaxed. In the elephant the same agencies, namely, 

 two muscles and an opposing elastic force, are employed, 

 though somewhat differently modified. 



" I may here briefly advert to the middle eye-lid of the 

 horse, one of the Pacchydermata : it consists of a true carti- 

 lage enclosed in a conjunctival fold, as in the elephant ; it is, 

 however, thinner and weaker, the pedicle is flat and thin, and 

 imbedded in fat ; this latter substance is collected into a large 

 oval mass, surrounded by a fine but distinct capsule, which is 

 connected to the adipose and cellular tissue in the orbit, and 

 which possesses some elasticity : an expansion from the su- 

 perior and internal recti muscles is also attached to it. The 

 eye in this animal is furnished with a powerful ' retrahens or 

 suspensorius muscle,' which extends from the optic foramen 

 around the optic nerve, and is inserted into the sclerotic coat. 

 When this muscle retracts the eye, the ball of fat slips for- 

 ward, and the cartilage and conjunctival fold shoot forward 

 and outward ; the eye at the same time is adducted by the 

 internal rectus, which also presses forward the fat ball and 

 the nictitating cartilage, and thus a great portion of the cor- 

 nea becomes covered or swept over by the third eye-lid, but 

 never to the same extent as in the elephant, where there are 

 the two proper nictitator muscles. In the latter, also, there 

 is no retrahens muscle, but the long and slender optic nerve 

 is surrounded only by cellulo-elastic tissue, which extends 

 from the apex of the orbit to a considerable bulbous swelling 

 around the entrance of the nerve into the eye ; this bulb ap- 

 pears composed of fibrous, cellular, and venous tissues. The 

 absence of this retrahens muscle in the elephant entails the 



