MOVEMENTS OF THE EYE : EYELIDS, ETC. 421 



object, whilst the head is inclined to one side or the other. 

 Of these muscles, the superior, inferior, and internal recti, 

 together with the inferior oblique, and also the elevator 

 muscle, i, of the upper eyelid, are supplied with motor nerves 

 by the third pair ( 459) ; whilst the superior oblique has a 

 nerve to itself, the fourth; and the external rectus has another 

 nerve to itself, the sixth. 



539. There is this very peculiar circumstance attending 

 the movements of the two eyes, that although they are 

 harmonious, they are seldom symmetrical. Thus, when we 

 direct our eyes towards an object on one side of us, they move 

 harmoniously, that is, with a common purpose ; but their 

 movement is not symmetrical, for one globe is rolled inwards 

 by the internal rectus, whilst the other is rolled outwards by 

 the external rectus. These two different actions seem to be 

 instinctively connected, and to be guided by the sensations 

 which are received through the two eyes respectively ( 478). 

 They are performed without any consciousness on our own 

 part, when, having fixed our gaze upon any object, we 

 rotate the head from side to side in the horizontal plane, the 

 eyeballs executing a corresponding rotatioin in the opposite 

 direction. 



540. The eyebrows, eyelids, and eyelashes, serve in various 

 ways for the protection of the eyes. In Birds and Reptiles 

 there is a third eyelid, which is drawn across the eye by a 

 muscle that passes through a loop in it. This nictitating 

 membrane, as it is termed, is semi-transparent ; and it serves 

 to protect the eye from the too-powerful rays of light, without 

 destroying the power of vision. The upper and lower eyelids 

 of Mammals, and the nictitating membrane of Birds and 

 Reptiles, are very frequently drawn over the front of the 

 globe during the waking state, for the purpose of sweeping 

 from it dust and other accidental impurities which would 

 otherwise be injurious. 



541. Beneath the upper eyelid, in the upper and outer 

 portion of the orbit, is situated the lachrymal gland (k, fig. 

 209) ; this is continually pouring-out a watery secretion over 

 the globe of the eye, which serves to wash from it these 

 impurities and to keep it moist. It is only, however, when 

 the quantity of this secretion is increased by mental emotion 

 or by irritation in the eye itself, so as to produce a flow of 



