152 HUMAN PHYSIOLOGY 



one for an opening serving such purposes and it is hard 

 to imagine why it should be a vertical slit in the cat and 

 horizontal in the horse. 



Behind the pupil is hung the dense but elastic body 

 which is called the crystalline lens. The name is some- 

 what unfortunate for it encourages the idea that this 

 is the only lens in the optic system. In reality it 

 has a smaller share in refraction than that borne by the 

 cornea. It is of interest most of all because it is ad- 

 justable. It is convex on both surfaces but less so in 

 front than behind. The presence of the lens separates 

 the interior of the eye into a smaller cavity between 

 it and the cornea and a larger one between it and the 

 retina. Both these spaces are filled by fairly clear 

 material, the aqueous humor before and the vitreous 

 humor behind the lens. 



The firmness of the eyeball is essential to its usefulness, 

 for any optical instrument must have a fixed form. 

 The eye resists deformation not so much because of the 

 strength of the outer coat as because of the high internal 

 pressure which prevails. This is derived indirectly from 

 the pressure of the blood in the arteries and the result 

 may be compared with the firmness developed in a 

 tire when it is inflated. When the pressure of the blood 

 falls at death the eye is soon soft and sunken. 



Through the combined effect of the cornea and the 

 crystalline lens a picture is made upon the retina. As 

 in any camera the image is upside down. It is very 

 foolish to make much of this fact, as people often do, 

 for there is no reason why we should not become ac- 

 customed to the order of things we have always known 

 and grow to regard the opposite relation as an inversion. 

 One of the first lessons learned by a baby is to reach 

 in a certain direction when an object makes a certain 

 retinal impression. If the image is high on the retina 

 he must reach down, while if it is low he must reach up. 

 Without knowing anything at all about the retina he 

 soon reacts unerringly. 



