XVII THE SENSE ORGANS 333 



tive surface of this layer is thereby stimulated and impulses 

 are carried through the optic nerve to the brain, there giving 

 rise to sensations of sight. The eye of the frog has no power 

 of accommodation for viewing both near and distant objects 

 such as our own eyes have. The lens cannot either be 

 changed in form or brought nearer to or farther from -the 

 retina, so that only objects are in focus which are at a particu- 

 lar distance from the eye. Images of objects at other dis- 

 tances are not clearly outlined, and there consequently 

 results imperfect vision. In the air the frog is myopic, or 

 near-sighted (Plateau, Hirschberg, Beer) ; i.e. it can see 

 clearly only objects which are near at hand. In the water, 

 on the contrary, it is hypermetropic, or far-sighted. The 

 myopia of the frog is evidently an advantage to the animal 

 in securing food, as it is important to see clearly objects 

 which are sufficiently near to be snapped at. It is evident 

 that the frog responds to the perception of movement much 

 more than of form, and for that purpose images which are 

 somewhat indefinite and blurred will suffice. Large moving 

 objects mean enemies, and consequently the necessity of 

 getting out of the way, even if there be no clear percep- 

 tion of the outline or the distance of the portentous thing 

 that is coming. 



The frog can be said to possess the power of binocular 

 vision only in a very small degree, since, owing to the fact 

 that the eyes are so laterally situated, their fields of vision 

 overlap anteriorly only to a very slight extent. 



The eyes may be moved in various directions by means 

 of special muscles. Four of these are the straight or recti 

 muscles, — a superior, an inferior, an anterior or median, 

 and a posterior or lateral rectus muscle, which roll the eye- 

 ball upward, downward, forward, and backward respec- 

 tively. There are two oblique muscles, the superior oblique, 



