THE SPECIAL SENSE ORGANS 



229 



The sneeze reflex is started either by a strong odor stimula- 

 tion or by a touch stimulation in some of the nerve endings in 

 the nostrils. Watering of the mouth in response to odors illus- 

 trates reflexes that are discharged to glands rather than to mus- 

 cles. The feeling of nausea and the act of vomiting are reflexes 

 .that may be started by 

 stimulation of the odor 

 end organs. 



In both taste and 

 smell the stimulation 

 depends upon the pres- 

 ence of particular sub- 

 stances in direct contact 

 with the delicate linings 

 of the end organs. These 

 materials must dissolve 

 and apparently pass 

 right to the nerve end- 

 ings by osmosis. 



188. Mechanical sen- 

 sitiveness. The varying 

 pressure of bones and 

 other structures, pres- 

 sure of food in the in- 

 testine, pressure of urine 

 inside the bladder, and 

 other contacts and pres- 

 sures within the body 



Fig. 115. The human ear 



A, the outer ear, consisting of the cartilaginous pro- 

 jection from the side of the head and an air pas- 

 sage, or vestibule, v; B, the middle car, lying 

 between the eardrum, or tympanum, t, and the 

 inner ear C. The inner ear is connected with the 

 pharynx by the Eustachian tube e (see Fig. 74). 

 Extending from the drum to the inner ear is a 

 series of three tiny bones : h, the hammer; a, the 

 anvil; and s, the stirrup. The main parts of the 

 inner ear constitute the labyrinth: c, the semi- 

 circular canals, consisting of three tubes placed 

 almost exactly at right angles to one another; k, 

 the cochlea, or snail shell. The labyrinth is filled 

 with a fluid and lined with a delicate membrane 

 containing nerve endings 



itself act as stimuli. 



Some of these touch or pressure stimuli start reflexes; others 

 start nerve discharges that end in our being aware of conditions. 

 Related to these, but depending upon special organs in the 

 inner ear, is the sensation of the position of the body and the 

 sensation of turning or spinning, which sometimes results in 

 dizziness. These organs consist of three tiny tubes, each in the 

 form of a half circle, arranged at right angles to one another 



