SENSE ORGANS 215 



The hydra, then, without a definite nervous system, ran 

 carry out a definite reflex action. 



Reflex action is similar in all animals. In all of these 

 illustrations, it is necessary for the stimulus to be received 

 by an afferent nerve, or some structure which can do the 

 same work, and for the stimulus to be transformed into a 

 series of purpose-like movements. 



166. Sense Organs. — All of the higher animals have 

 eyes, ears, a nose, and a tongue. Each of these organs 

 contains nerves specialized to respond to a certain definite 

 kind of stimulus. The result of this specialization is that 

 not only are these special sense organs complex: in struc- 

 ture, but also the region of the brain which receives their 

 messages. The ear nerve responds to a stimulus of 

 air-waves of a certain length, and we say we hear a 

 sound. The eye nerve is stimulated only by light. 

 Each nerve and the brain cells to which it sends its 

 messages have become so specialized that practically 

 only one kind of reaction takes place. For example, 

 all stimuli acting upon the eye nerves are interpreted 

 as light. 



The skin is a simpler sense organ than the eye or ear. 

 and tells us of pain and touch and the difference between 

 heat and cold. 



The Eyes. — The eyes of all vertebrates have the parts 

 arranged in a similar manner. The eyeball is roundish 

 and is located in the eye sockets of the skull, which are 

 termed orbits. There is an upper and a lower eyelid, and 

 the remains of a third eyelid in the corner next to the 

 nose. The front of the eve is covered by a transparent 

 membrane, the cornea (kor'ne-a); and the rest of the 

 eye is surrounded by a tough membrane, the Bclerotic 

 coat, or the white of the eye. Within the combined 

 covering of the cornea and sclera are a number of struc- 



