MOVEMENTS OF THE EYE :—EYELIDS, ETO. 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, 7, 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 
harmoriously, 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 
ae 
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 
