PROTECTIVE MECHANISMS OF THE EYEBALL 97 



muscle of Gegenbauer" (Fig. 23). In the Primates, who 

 have no third eyelid or choanoid muscle, there is an outer 

 bony wall to the orbit, the only gap in its continuity being 

 the sphenomaxillary fissure. 



The retraction of the eyeball by the choanoid muscle 

 causes the contents of the orbit to become pressed against 

 the fibro-muscular membrane, filling in its outer wall. It 

 is partly the elastic recoil of this membrane, and partly the 

 contraction of Gegenbauer 's muscle, which restores the 

 eyeball to its usual position, and brings about the retraction 

 of the third eyelid. 



In association with the third eyelid there is a special 

 gland, "the Harderian gland," whose duct opens on its 

 inner surface, and whose secretion is emptied when the 

 eyelid is pressed forward, forming a lubricating fluid for 

 the surface of the cornea. Among mammals this gland is 

 largest in the Ungulata, whose third eyelid is most mobile, 

 smaller in the Carnivora, and absent altogether in monkeys 

 and man who have no third eyelid. 



Some mammals possess both a lacrimal gland and a 

 Harderian gland; where, however, the latter is large and 

 the third eyelid is efficient the former is usually small. In 

 monkeys and man, who have no third eyelid or Harderian 

 gland, the lacrimal gland is larger than in other mammals, 

 and largest of all in man. 



The functions of the third eyelid are obvious from its 

 anatomical arrangements, it protects the front of the eye 

 from injurious influences, lubricates its surface, and removes 

 any foreign substance which may become lodged on it. In 

 animals, such as the Herbivora, who when feeding hold 

 their heads close to the ground, and in the Carnivora, who 

 do the same in tracking their prey by scent, the presence 

 of such a protective mechanism is of the greatest service 

 7 



