472 
THE NERVOUS SYSTEM OF MAN 
Reflex action is similar in all animals. In all these il¬ 
lustrations, 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 purposeful movements. 
364. Sense Organs. — All the higher animals have eyes, 
ears, a nose, and a tongue. Each of these organs con¬ 
tains 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 be¬ 
come so specialized that practically only one kind of reac¬ 
tion 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. 
Smell and Taste. — These two senses are closely related. 
The sense of smell is located in the nose and the organs of 
smell are minute nerve cells scattered among the regular 
cells that line the nasal passageway. The olfactory nerve 
which carries smell stimuli to the brain is the shortest nerve 
in man. 
Taste has already been described in connection with the 
digestive system on page 405. 
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 eye is covered by a transparent mem- 
