THE WONDERS OF LIFE 



that the odorous matter, or olfactory stimuH, be brought 

 in a finely divided condition over the moist olfactory 

 membranes. When they touch the olfactory cells — 

 slender, rod-shaped cells with very fine hairs at the free 

 end — they excite the ends of the olfactory nerve which 

 are connected with the cells. 



In many animals, especially mammals, the sense of 

 smell has a much more important part in life than it has 

 in man, in whom it is relatively feeble. It is well 

 known that dogs and other carnivora, and even ungulates, 

 have a much keener smell. In these cases the nasal 

 cavity, which is the seat of the sense, is much larger, 

 and the muscles in it are much stronger. The nostrils 

 of the air-breathing vertebrates have been developed 

 from a pair of open nasal depressions in the skin of the 

 fish's head. But in these aquatic vertebrates the 

 chemical action of the olfactory stimuli must be of a 

 different character, like the sensation of taste. The 

 odorous matter is, in these cases, brought into contact 

 with the olfactory membrane in a liquid form (in which 

 condition it is not perceptible to man). In fact, the 

 division between the senses of smell and taste disappears 

 altogether in the lower animals. These two "chemical 

 senses" are closely related, and have a common feature 

 in the direct chemical action of the stimulus on the 

 sensitive part of the skin. 



A chemical sensation of matter that corresponds 

 completely to the real taste-sensation in the higher 

 animals is found in some of the higher carnivorous 

 plants. The leaves of the sun-dew (drosera rotundifolia) 

 are very sensitive insect-traps, and are armed at the 

 edge with knob-like tentacles, sticky hairs that secrete 

 an acid, flesh-digesting juice. When a solid body (but 

 not a raindrop) touches the surface of the leaf the 

 stimulus acts in such a way on the tentacle heads as to 

 contract the leaf. But the acid fluid which serves for 



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