188 ANATOMY OF THE HEAD. 



parts being furnished from the maxillary ; and the internal by 

 the ophthalmic, and some branches that penetrate the scle- 

 rotic coat ; and the veins return their blood to the sinuses 

 within the cranium, so to the jugulars. The optic nerve 

 enters the orbit, and, piercing the globe, forms the retina, 

 having the arteria centralis retina given off from the brain 

 within its centre : the eye receives nervous influence also 

 from the second, third, fourth, fifth, and sixth nerves ; of 

 which the fifth alone endows it with sensation. The second 

 is a special nerve of vision ; all the rest are for motion. 



PHYSIOLOGY OF THE EYE, OR THE FUNCTION OF VISION. 



Light is the appropriate stimulus to the functional pur- 

 poses of the eye ; enabling it to take cognizance of the 

 figure, colour, and distance of bodies. The visual rays, in 

 this climate, shoot rectilinearly through space : but when 

 they pass through denser media, they do not preserve this 

 rectilinear course ; but are bent in a degree equal to the 

 density of the medium through which they pass. This 

 bending of the rays is called their refraction : the tendency 

 of which is to bring them to ti focus ov focal point ; to which 

 point all luminous rays reach, sooner or later, according to 

 the increased or diminished density of the medium they 

 have to pass through ; upon which property most of the 

 phenomena of vision depend. This premised, it will become 

 evident that the visual rays meet with several different re- 

 fractions, or bendings, in passing through the eye. Their 

 course through the cornea and aqueous humour must form 

 their first refraction ; that through the crystalline lens will 

 be the next, the powers of which, as a refracting medium, 

 must be great ; and in their further course through the 

 vitreous humour, and the repeated folds of its membrane, 

 they must undergo a still farther bending ; till they meet in 

 a point on the retina : thus forming a cone, the basis of 

 which will be the surface of the cornea, and the apex the 

 retina. The rays of light fall upon an object, or upon 

 several objects. They are thence reflected in such a form 

 as conveys to the spectator a sense of colour upon entering 

 the eye ; they are so refracted that a perfect image, though 

 infinitely small, is painted upon the retina. 



Amidst the wonderful number of objects that present 



