396 



ORGAN SYSTEMS OF MAN 



should according to its predetermined func- 

 tion. There are some classes of substances 

 which have a consistent taste, for example, 

 acidic substances usually taste sour, basic 

 substances bitter. The threshold (a stimu- 

 lus that is just sufficiently strong to elicit a 

 response) is very low for the sense of both 

 taste and smell. For example, it is possible 

 to taste quinine in concentrations of one 

 part in two million, and much greater dilu- 

 tions of odorous substances can be smelled. 



Smell. The olfactory end organs which 

 are responsible for the sense of smell are 

 located in the nasal membranes, and it is 

 through these organs that gaseous chemical 

 stimuli (odors) are received. While in man 

 the receptive area in the two nasal chambers 

 is only about 10 square centimeters, in most 

 mammals it is much more extensive. It will 

 be recalled that the sense of smell is far 

 more important to ground dwellers than to 

 those that live in trees, where keen vision 

 is of more value. It is not surprising, there- 

 fore, that when the primates took to the 

 trees the sense of smell diminished and in 

 the present primate is very poorly devel- 

 oped. The dog, on the other hand, receives 

 a great deal of information about the world 

 through his nose. This sense is so keen that 

 the dog can pick up the odor of an animal 

 that has passed over a trail some hours 

 before. Most game dogs rely more on their 

 sense of smell than on sight except at close 

 range, and, in fact, many bird dogs are very 

 near-sighted. 



The olfactory cells give rise to fibers 

 (Fig. 16-2) which coalesce, after passing 

 through the cribiform plate (p. 378), to 

 form the olfactory bulb, which becomes 

 a large nerve leading to the brain. The 

 location of the olfactory end organs is such 

 as to protect them from the desiccating ef- 

 fects of incoming currents of air during 

 respiration, and the nasal passages are kept 

 continuously moist in order that the incom- 

 ing odors may dissolve in the fluid bathing 

 them. A dry olfactory end organ cannot 



be stimulated any more than can a dry 

 tastebud. 



It may be necessary for large quantities 

 of air to pass over the receptors before 

 stimulation is possible. As more air passes 

 over them more of tlie chemical in gaseous 

 form becomes dissolved in the fluid of the 

 nose, thus increasing the concentration to 

 a point where the threshold is exceeded. 

 This accounts for the constant sniffing of 

 the dog, or man, too, on occasion, to bring 

 more air into contact with the nasal epi- 

 thelium. 



Our knowledge of the sense of smell is 

 very limited, as indicated by the fact that 

 no satisfactory system of classification of 

 odors has yet been set up. Odors are still 

 referred to by the name of the aromatic 

 substance in question, and there are nearly 

 as many names for odors as there are aro- 

 matic chemicals. It seems highly improb- 

 able that there is an infinite number of 

 kinds of olfactory nerve endings, although 

 when they become fatigued to one odor 

 they seem to respond with normal vigor to 

 another. Different chemicals apparently do 

 not stimulate the same nerve endings. In 

 general, the nerve endings fatigue readily, 

 and it is a familiar experience that an odor 

 which is very strong when one first enters a 

 room soon fades away until it is unnoticed, 

 not because the chemical in the air has 

 diminished in quantity but because the 

 olfactory end organs fail to be stimvilated 

 beyond a certain brief period. Just why this 

 is so is not clearly understood at present. 



Vision 



This is the most perfect of the distance 

 receptors. It provides us the means of keep- 

 ing aware of our environment at small or 

 great distances, whether reading this page 

 or looking at the stars. Not only is this sense 

 the most important for man but for all pri- 

 mates, as well, and most of their informa- 

 tion concerning their environment comes 

 through this sense. 



