CHAPTER XXXII 



THE SENSE-ORGANS 



The Eye. — With regard to the eyes, two points of interest 

 present themselves. The first concerns the method of 

 accommodation of the eye for seeing objects at different 

 distances, and the second relates to the capacity of some 

 animals to see a single object with both eyes at the same time. 



Accommodation is a simple optical problem concerning 

 the focal length of the lens, the distance of the viewed object, 

 and the distance between the lens and the retina. These 

 three terms must be in relation according to the laws of optics 

 if there is to be a clear image of the object on the retina. The 

 first and the third term are within the animal, and are there- 

 fore variable, while the second, the distance of the object, is 

 obviously external to the animal and not under its direct 

 control. It is found that some animals accommodate by alter- 

 ing the distance between the lens and the retina, and others by 

 altering the focal length of the lens itself. 



Cyclostomes and Selachians may be left out of account, for 

 their eyes can accommodate but little if at all. In the bony 

 fish, the eye when at rest is accommodated for near vision. 

 This fact is in relation to the optical nature of the medium in 

 which they live, water, through which it is not possible to see 

 very far. The lens is attached to the eye-cup by a retractor 

 lentis muscle, and when this contracts, the lens is brought 

 nearer to the retina, and the eye can then focus objects which 

 are farther away. Land-vertebrates always have their eyes 

 focussed at rest for distant vision, which enables them the 

 earlier to see their prey or their enemies. So, in amphibia, 

 the lens is attached to the eye-cup by a protractor lentis muscle. 



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