Chap. 17 



RESPONSIVENESS THE SENSE ORGANS 



315 



Olfactory 

 hairs 



sustentacular 

 cells 



sensory 

 cells 



basal 

 eel I s 



Fig. 17.4. Human olfactory organ. A, side wall of nasal chamber with the pro- 

 jecting turbinate bones and the clefts between them. Arrows indicate the course of 

 air in ordinary breathing. B, sniffing the air brings it forcibly against the olfactory 

 organ located under the lobe marked by the circle of arrows. C, diagram of sensory 

 receptor cells of smell with their supporting cells (sustentacular). The sensory cell 

 has a single dendrite which extends to the exposed surface where it is expanded 

 into a bulb which bears delicate processes, the olfactory hairs. These processes ex- 

 tend into a film of mucus that covers the surface of the organ. Extremely minute 

 particles of substance inhaled, whether skunk, garlic, or lily fall into the fluid and 

 upon the ends of the olfactory cells. A chemical change immediately occurs, passes 

 through the sensory cell, and by way of an olfactory nerve to the cells in the brain 

 and the interpretation of the odor. {A and B after Biology: Its Human Implications, 

 2nd ed. by Garrett Hardin. Copyright, 1952. W. H. Freeman and Company. C 

 after Smith, Canadian Med. Assn. J., 1939.) 



the positions of these and keep them where they belong. This is brought to 

 them through the statoreceptors. 



The majority of active multicellular animals have these paired organs of 

 equilibrium, of essentially the same structure wherever they occur. A stato- 

 receptor is a more or less spherical sac containing fluid and freely movable 

 granules, the statoliths. Minute bristles that project into the fluid are attached 

 to sensitive cells in the wall of the sac, and these in turn touch the nerve 

 fibers. The statolith is attracted by gravity; it rolls about, always resting on 

 the downside, and its pressure upon the bristles is the stimulus of the receptors. 



