234 



ELEMENTARY BIOLOGY 



a 



By seeing we mean not simply discriminating between light 

 and dark, but being capable of distinguishing forms and 

 colors, as well as light and shade, at some distance. 



276. The range 

 of sensitiveness. 



Some insects, as 

 ants, have shown 

 that they are sensi- 

 tive to other vibra- 

 tions that make no 

 impression at all up- 

 on our retina. We 

 can understand this 

 both from our ex- 

 perience with other 

 senses and from ex- 

 periments. Thus, we 

 know that a blood- 

 hound will percei\e 

 odors that you and 

 I would pass by 

 without notice, and 

 that even most dogs 

 would not be able 

 to recognize. We 

 know that ordinary 

 dogs will recognize 

 persons by the odor, 

 but that very few human beings are capable of doing so. Again, by 

 means of experiments we have found out that some people can dis- 

 tinguish shades of red or of blue that others cannot recognize at all. 

 We can therefore understand that some nerve end-organs will be sen- 

 sitive to certain colors (rates of vibration) that leave others indifferent. 

 Moreover, some animals are sensitive to light of such low intensity 

 as others would not respond to at all, just as individuals vary as to 

 the quantity of light stimulation necessary to make an impression. 



^^>\%^^^ 



Fig. 9S. The third eyelid 



The little fold of tissue extending from the inner corner of 

 the eye corresponds to the third eyeUd, or iiictitathig mcni- 

 b7'an€, in birds and certain reptiles and amphibians. The 

 nictitating membrane can be drawn over the eye so as to 

 cover it completely, a, eye of ape : /;, eye of owl ; r, human 

 eve : i\, the semilunar fold, eyeball removed 



