144 ANLMAL SENSE ORGANS 



vibrations of the ear-drum are transmitted to a liquid 

 that is within the sensitive part of the ear, setting the 

 liquid in turn into vibration, and the vibrations of the 

 liquid affect protoplasmic receptors which are directly 

 exposed to them. 



Sight. — This sense is an extension, amplification, and 

 modification of the response of protoplasm to light, de- 

 scribed in Chapter XII. In its simplest form in animals 

 it consists, as stated in the beginning of the present 

 chapter, of an effect upon the protoplasm of such sort that 

 the animal is impelled to move either toward or away 

 from the light. This implies both reaction to the pres- 

 ence of light and some kind of recognition of the direc- 

 tion from which it comes. There are very few kinds of 

 animals that are devoid of this type of sensation. The 

 sense of sight, as commonly thought of, includes, along 

 with the perception of light, recognition of the form of 

 objects. As soon as perception of form is achieved a 

 great advance is made in the utility of the sense, because 

 there is now the possibility of gaining accurate infonna- 

 tion as to the nature, size, direction, and distance of the 

 objects that make up the environment. This important 

 knowledge cannot be obtained with anything like the 

 same degree of accuracy by all the other senses working 

 together. 



The Perception of Form requires that the object shall 

 reflect light upon a sensitive protoplasmic surface, and 

 that the light so reflected shall make upon this surface 

 a pattern or image of the object from which the light 

 comes. In a strict sense, any specialized receptor which 

 reacts to light is an eye, but in all the higher animals and 

 in many of the lower as well the eye is an apparatus for 

 forming images. 



Image Formation is accomplished in the eyes of ani- 

 mals in at least two different ways; the compound eyes 

 of insects do it in one way, while the eyes of vertebrate 

 animals — including man — do it in another (Fig. 38). 



