98 CONSCIOUS NERVOUS OPERATIONS 



odoriferous particles reach the olfactory cells. Thus 

 one is able to examine more fully the odors of the air, 

 and several different " smells " may be sometimes distin- 

 guishable at once. Usually the odors reach both nostrils 

 at the same time and two impulses are conducted along 

 the two olfactory nerves ; they are, however, fused into 

 one sensation. If different odors are brought at one 

 time to the olfactory cells of the two nasal passages, one 

 sensation sometimes destroys the other ; sometimes first 

 one and then the other odor is perceived ; in any case 

 there is but one sensation. 



128. Other afferent impulses than those of smell may 

 arise in the nasal membrane. A very pungent substance, 

 such as ammonia, causes sensation distinct from smell, 

 sensation which is found to belong both to parts of the 

 nasal membrane on which the olfactory nerves ramify 

 and also to other portions. 



A very small quantity of odoriferous material is suffi- 

 cient to excite the sensation of smell. A very minute 

 particle of musk, for instance, will fill a large room with 

 its odor, and that for an indefinitely long time. 



The end organs of the olfactory neryes are soon ex- 

 hausted, and sensation dies out. We soon cease to notice 

 the odors in a room, though we may have thought them 

 overpowering on entering. Many animals are much more 

 liberally endowed with the power of detecting and discrim- 

 inating odors than is man. 



129. Path of an Olfactory Impulse. The olfactory cells 

 in the epithelium of the nasal passages send a process 

 to the surface of the mucous membrane, and another 

 inward (Fig. 57). The latter process of each cell ends 

 in fine spreading fibrils which mingle with similar brush- 

 like fibrils from a deeper layer of nerve cells in the olfac- 



