160 BIOLOGY: GENERAL AND MEDICAL 



etc. But such development is not possible until there 

 appear in the superficial tissues highly specialized nerve 

 endings (Meissner's corpuscles) adapted to the recep- 

 tion of the particular impressions, and in the central 

 nervous system that complex adjustment of white and 

 gray matter by which the impressions are received and 

 appreciated. 



Sight. If we define sight as the ability to appreciate 

 light, we must admit that it makes its appearance in the 

 most simple form as the phototropic sensitivity of 

 protoplasm. If, however, we mean by the term, the 

 recognition of light by the aid of an eye, we are almost 

 as badly off because of the uncertainty as to what shall 

 constitute an eye. In its broadest sense, an eye is an 

 organ formed for the appreciation of light, to the waves 

 of which it is specially sensitive. Such an organ may 

 be extremely simple in structure, totally devoid of the 

 power of forming images of external objects, and con- 

 sist merely of a group of pigmented cells, as for example, 

 the eye-spot of Euglena or the eyespots in the higher 

 coelenterates. The latter connect with subjacent nerve 

 cells, thus forming a true sensory organ, though in what 

 manner the impressions received are utilized is difficult 

 to understand. 



Similar eyes are also found in the flat worms, though in 

 these the nervous elements become more numerous, and 

 instead of being upon the surface of the body, the organ 

 becomes situated in a more or less pronounced pit or 

 depression from which the nervous cells radiate, the 

 fibres with which they are connected converging to 

 form an optic nerve. Eyes of this kind persist among 

 the arthropods, though more highly specialized eyes 

 are also present. 



From such eyes it is a simple step to the camera-eye, in 

 which the nervous elements surround a spherical space 

 into which the light comes through a minute opening, 

 the homologue of the pupil, causing an inverted image 

 to fall upon the more numerous nerve elements and 



