PRODUCTION OF SENSATIONS 347 



resemble closely the cell-bodies of neurons, and are thought 

 to be such. The divisions of the olfactory nerve pass 

 through many small openings in the ethmoid bone to 

 connect with the olfactory bulbs, which in turn connect 

 with the cerebrum (A, Fig. 147). 



The Olfactory Stimulus. Only substances in the gas- 

 eous state can be smelled. From this it is inferred that the 

 stimulus is supplied by gas particles. Solids and liquids are 

 smelled because of the gas particles which separate from 

 them. The substance which is smelled must be kept mov- 

 ing through the nostrils and made to come in direct contact 

 with the olfactory cells. There is practically no limit to 

 the number of distinct odors that may be recognized. 



Value of Smell. Although the sense of smell is not so 

 acute in man as in some of the lower animals, it is, never- 

 theless, a most important and useful gift. It is the only 

 sense that responds to matter in the gaseous state, and is, 

 for this reason, the only natural means of detecting harm- 

 ful constituents of the atmosphere. In this connection it 

 has been likened to a sentinel standing guard over the air 

 passages. Many gases are, however, without odor, and 

 for this reason cannot be detected by the nostrils. It is of 

 especial importance that gases which are likely to become 

 mixed with the air supply to the body have odor, even 

 though the odor be disagreeable. ' The bad odors of 

 illuminating gas and of various compounds of the chemical 

 laboratory, since they serve as danger signals to put one 

 exposed to them on his guard, are of great protective value. 



Sight and Hearing. The sense organs of sight and 

 hearing are highly complicated structures, and will be 

 considered in the chapters following. 



Summary. Sensations are certain activities of the 

 mind that result from excitations within the body or at its 



