THE PROTECTED MECHANISMS OF THE EYE. 789 



apices not falling on exactly corresponding parts would give rise to two 

 perceptions, and the whole object ought to appear confused. That it does 

 not, but, on the contrary, appears a single solid, must be the result of cere- 

 bral operations, resulting in what we have called a judgment. 



692. Struggle of the two fields of vision. If the images of two surfaces, 

 one black and the other white, are made to fall on corresponding parts of the 

 eye, so as to be united into a single perception, the result is not always a 

 mixture of the two impressions, that is, a gray, but, in many cases, a sensa- 

 tion similar to that produced when a polished surface, such as plumbago, is 

 looked at : the surface appears brilliant. The reason, probably, is because 

 when we look at a polished surface, the amount of reflected light which falls 

 upon the retina is generally different in the two eyes ; and hence we associate 

 an unequal stimulation of the two retinas with the idea of a polished surface. 

 So, also, when the impressions of two colors are united in binocular vision, 

 the result is, in most cases, not a mixture of the two colors, as when the same 

 two impressions are brought to bear together at the same time on a single 

 retina, but a struggle between the two colors, now one and now the other, 

 becoming prominent, intermediate tints, however, being frequently passed 

 through. This may arise from the difficulty of accommodating at the same 

 time for the two different colors (see p. 758); if two eyes, one of which is 

 looking at red and the other at blue, be both accommodated for red rays, 

 the red sensation will overpower the blue, and vice versa. It may be, how- 

 ever, that the tendency to rhythmic action, so manifest in other simpler 

 manifestations of protoplasmic activity, makes its appearance also in the 

 higher cerebral labors of binocular vision. 



THE PROTECTED MECHANISMS OF THE EYE. 



693. The eyeball is protected by the eyelids, which are capable of move- 

 ments called respectively opening and shutting the eye. The eye is shut by 

 the contraction of the orbicularis muscle, carried out either as a reflex or 

 voluntary act by means of the facial nerve. The eye is opened chiefly by 

 the raising of the upper eyelid through the contraction of the levator pal- 

 pebrse carried out by means of the third nerve. The upper eyelid is also 

 raised and the lower depressed, the eye being thus opened, by means of 

 plain muscular fibres existing in the two eyelids and governed by the cer- 

 vical sympathetic. The shutting of the eye, as in winking, is in general 

 effected more rapidly than the opening. 



The eye is kept continually moist partly by the secretion of the glands 

 in the conjunctiva, and of the Meibomian glands, but chiefly by the secre- 

 tion of the lachrymal gland. Under ordinary circumstances the fluid thus 

 formed is carried away by the lachrymal canals into the nasal sac and thus 

 into the cavity of the nose. When the secretion becomes too abundant to 

 escape in this way it overflows on to the cheeks in the form of tears. 



If a quantity of tears be collected, they are found to form a clear, faintly 

 alkaline fluid, in many respects like saliva, containing about 1 per cent, of 

 solids, of which a small part is proteid in nature. Among the salts present 

 sodium chloride is conspicuous. 



694. The nervous mechanism of the secretion of tears, in many re- 

 spects, resembles that of the secretion of saliva. A flow is usually brought 

 about either in a reflex manner by stimuli applied to the conjunctiva, the 

 nasal mucous membrane, tongue, optic nerve, etc., or more directly by 

 emotions. Venous congestion of the head is also said to cause a flow. The 

 efferent nerves belong either to the cerebro-spinal system (the lachrymal and 



