164 THE SENSES. 



one-twentieth of their length. The cones are very similar, except 

 that their conical shape makes them appear to be of different cha- 

 racter. The cones do not always reach to the same level as the 

 rods. When viewed from the retinal surface, the ends of the rods 

 and cones give the appearance of a fine mosaic. These organs are 

 connected with the subjacent nervous tissue, but the mode of their 

 connection with the optic nerve is not fully understood. 



Considered as an apparatus, how is the eye arranged? 



It may be compared to the photographic camera. It contains vari- 

 ous ^media for the refraction of light, and a screen at the back for 

 receiving the image. The refracting media are the cornea, aqueous 

 humor, crystalline lens, and the vitreous humor : the screen is the 

 retina. The pigment of the retina and choroid makes the interior 

 dark, a necessary feature in such an apparatus. The mechanism of 

 the lens enables the eye to be focused for distance, while the iris 

 regulates the proper admission of light. 



What is the structure of the cornea ? 



It is continuous with the sclera in the front of the eye, and 

 occupies about one-sixth of the surface (3, Fig. 47). Its shape is 

 that of a small dome set upon the globe of the eye. It has in 

 front several layers of epithelial cells, and at the posterior surface 

 a thin epithelial lining (membrane of Descemet) ; but the main 

 body of the structure consists of interlacing connective-tissue fibres, 

 which have spaces in which are found branching cells peculiar 

 to the structure. The cornea is perfectly transparent : it has no 

 blood-vessels. 



Describe the crystalline lens. 



It is a double convex lens (8, Fig. 47) of high refractive power 

 which is suspended in the anterior portion of the eye immediately 

 behind the anterior chamber. Its function is to bring the rays of 

 light to a focus upon the retina. In structure the lens is com- 

 posed of concentric layers of long, slender fibres enclosed in a thin 

 capsule. 



What are the aqueous and vitreous humors ? 



(1) The aqueous humor is a watery fluid contained in the 

 anterior chamber. It acts with the cornea as a refracting medium 

 to concentrate rays of light upon the lens, to maintain the globular 

 form of the cornea, and to float the iris and allow it freedom of 

 motion. 



