98 EVOLUTION OF THE HUMAN EYE 



in guarding the cornea from coming in contact with foreign 

 substances, or in their removal should they gain access to 

 the eye. 



The grasping of the food in the first place with the hands 

 and the conveyance of it by them to the mouth, as is done 

 by animals who have adopted arboreal life, saves the eyes 

 from much of the risk to which they are exposed when, as 

 in grazing, the food is seized directly with* the mouth. The 

 development of the forelimbs into organs of delicate tactile 

 sensibility, capable of picking up minute particles, has 

 made it possible for them to be used for the removal of 

 foreign bodies from the eye, and thus, as suggested by 

 Le Coq, to play the part of the third eyelid in a way which 

 is not possible for hoofs or claws. 



The greater flow of lacrimal secretion in response to the 

 irritation of foreign bodies supplies also an hydraulic 

 apparatus for removal of foreign bodies, to replace the 

 mechanical arrangement of the third eyelid. 



Not only as the third eyelid becomes a superfluous structure 

 in Primates, but the retraction of the eye which its use entails 

 would be a source of trouble and inconvenience to them. 

 The movement of the eyes backward and forward in the 

 orbit in animals who have assumed the erect or semierect 

 posture would considerably interfere with the accuracy of 

 stereoscopic vision and of accommodation and convergence. 



The vestigial remains of the third eyelid persist in man 

 and monkeys as the semilunar fold. Owen 48 says that a 

 remnant of the choanoid muscle is to be found in the lower 

 Quadrumana, in the form of a few fibers detached from the 

 inner part of the recti to be inserted into the sclerotic nearer 

 the entry of the optic nerve. Sir John Bland-Sutton 49 has 

 recently suggested that Tenon's capsule is really the repre- 

 sentative of this muscle in man. He says: "A critical 



