804 



THE SENSE OF SIGHT 



numerous funnel-shaped tubules, through which the rays of light are 

 refracted by means of a lens-like structure of chitin. This type of eye, 

 however, is soon abandoned, because already in the cephalopods we 

 find a single system of curved refracting media. In the vertebrates 

 the eye is constructed along very similar lines. Retrogressive it 

 becomes in proteus and sphalax, because these animals live perma- 

 nently in the dark. 



The eye is the organ of space, its purpose being to form images 

 of external objects upon the retina which are then conveyed into 

 consciousness. Its general structure and manner of action reminds 



Fio. 420. DIAGRAM OF A HORIZONTAL SECTION THROUGH THE HUMAN EYE. 



C, cornea; A, anterior cavity; P, posterior cavity; L, lens; J, iris; T, conjunctival sac; 

 CL, ciliary ligament; CB, ciliary body; CM, ciliary muscle; OS, ora serrata; CS, canal of 

 Schlemm; R, retina; Ch, choroid; S, sclera; ON, optic nerve; A, retinal artery; B, blind 

 spot; Y, yellow spot; OA, optical axis; VA, visual axis; H, hyaloid canal. 



us of the camera obscura, the box of which is represented by the cor- 

 neal and sclerotic envelope of the eyeball, its refracting medium by the 

 aqueous humor, lens and vitreous humor, its diaphragm by the 

 iris, and its sensitive screen by the retina. Its most essential constitu- 

 ent is, of course, the retina, while its other structures merely serve the 

 purpose of adjuncts to effect a proper concentration of the rays of light. 

 The eyeball is spheroid in shape and is loosely held in the orbital 

 cavity by a fibrous membrane, known as the capsule of Tenon. Its 

 anteroposterior diameter measures about 24 mm., and its transverse 

 and vertical diameters about 22 mm. In longitudinal section it is 

 seen to be composed of the segments of two spheres, of which the pos- 



