Nervous System and Sense Organs 215 



in reality only odors ; for example, vanilla has no perceptible flavor 

 but has a definite odor. Chloroform, on the other hand, is a taste and 

 not an odor. 



The Eye. — Of all the sense organs which help interpret the en- 

 vironment, the eye with its reception of visual stimuli ranks near the 

 top in importance (Fig. 67). The sense receptors for vision are located 

 in the eyeball and are able to function so efficiently because of the many 

 accessory structures. These many accessory structures make the eye 

 one of the most complex of the sense organs. 



SCLEROTIC COAT 



CHOROIO COAT 



RETINA 



CILIARY BODY. 



Fig. 67. — Section through eye. 



The eyeball is well protected within its bony socket in the skull. 

 In human beings, the supraorbital ridge of the frontal bone projects 

 slightly over the eye. On this ridge is a fleshy fold provided with 

 hairs, the eyebrow, which also may have some protective function. Be- 

 low, the malar bone and part of the maxilla form a ventral protection. 

 Only the anteriormost portion of the eyeball is left relatively unpro- 

 tected, but inasmuch as it is set so far back, accidental injuries are 

 relatively rare. Further, considerable flexibility is attained by the pres- 

 ence of fibrous membranes which hold the eyeball loosely in place within 

 the bony socket. 



The eyeball toward the front is given additional protection by several 

 special structures. Most conspicuous of these are the tough eyelids, 

 folds of skin which are able to close over the eye. The reflex for clos- 

 ing these lids is a very strong one, and will occur when an object is 

 simply passed close to the eye. The borders of the lids are equipped 



