THE SPECIAL SENSES. 137 



The sixth is the opponent of the fifth. It is a short muscle 

 arising from the socket near the tear sac, and, passing beneath 

 the eye-ball, is attached to its outer side. The oblique mus- 

 cles rotate the eye as the head is turned, in order that 

 objects may be pictured on the same part of the retina. 

 When acting with the straight muscles, they move the eye 

 in oblique positions. For instance, the eye can be turned 

 outward and obliquely upward. 



Through the combined action of these six muscles the 

 various movements of the eye are performed. The wonder 

 of this exquisite mechanism is greatly increased when it is 

 remembered that the two eyes act together and are moved 

 in the same direction at the same moment. This harmony 

 in the action of the muscles of both eyes is maintained by 

 the close connection between the nerves supplying these 

 muscles. If from want of proper nerve supply, or from 

 weakness or disease, any one of the muscles acts imperfectly, 

 the effect is at once noticeable in the nature of a " cast," or 

 "squint." 



15. How we See. To have a thorough understanding of 

 the eye as an optical instrument, it would be necessary to 

 have a clear knowledge of the properties of light. It must 

 here suffice to say that the light, in the form of undulating 

 rays, passes through the transparent cornea, then through the 

 pupil in quantity according to its size, and falling upon the 

 lens, converges to a point at the retina. 



The rays of light strike the convex cornea from different 

 directions. Those which are straight for its centre pass 

 through without any deviation. Of those which come from 

 the different angles, some are too near the outer border of the 

 cornea and are reflected back, while others pass in, and are 

 more or less refracted, or brought nearer together. The 

 humors of the eye also bring the rays nearer, but the lens 

 is the chief instrument of refraction. By reference to Fig. 



