240 THE SPECIAL SENSES 



necessary. It is of the greatest importance to animals, and 

 therefore several mechanisms share in its performance. It is 

 brought about by sensory impulses coming from many sources, 

 and the sum total of the sensations involved constitutes the 

 sense of equilibrium. Every known sensation probably contrib- 

 utes to the maintenance of equilibrium, but certain structures 

 in the ear, known as the semicircular canals, form the main 

 source. When the canals are injured in any way, the animal 

 sprawls on the ground, holds its head in an unnatural position, 

 makes peculiar forced movements, etc. The effect varies with 

 the number and the position of the canals operated upon, and 

 ranges from simple unsteadiness of gait to complete inco- 

 ordination. These results are explained as being due to increased 

 or decreased pressure of the endolymph on the cristse acusticse 

 of the ampullse of the semicircular canals. The latter are 

 situated in the three dimensions of space, and rotation of the 

 body in any direction can be judged quite accurately as to 

 direction and amount. Rapid rotation in one direction is, after 

 its cessation, often followed by a sensation of rotation in the 

 opposite direction. Excessive rotation leads to dizziness, but 

 in deaf mutes dizziness is difficult to produce on account of the 

 imperfect development of the ears. Diseases which alter the 

 pressure in the canals lead to vertigo and incoordination. 

 The semicircular canals in themselves, however, are not suffi- 

 cient to preserve complete equilibrium. 



Smell. -The special olfactory mucous membrane is situ- 

 ated in the upper part of the nasal cavity, away from the 

 direct current of the inspired air. The rod cells which it con- 

 tains are connected with fibers that are part of the olfactory 

 nerve and constitute the sensory nerve endings. Substances 

 that excite the sense of smell are in a fine state of division, or 

 in a gaseous state, and are brought into contact with the rod 

 cells by rapid but short inspiratory movements. They are 

 perceived best when the air is at the body temperature. The 

 substance producing smell is probably taken into solution in 

 the moisture covering the olfactory membrane. In the lower 

 animals the sense of smell plays a very important part, and it is 

 probable that all animals give out characteristic odors by which 

 they recognize one another. The sense of smell is, therefore, 

 highly developed, which is not the case in man. The distri- 



