768 PHYSIOLOGY. 



Auditory Sounds. All auditory sensations are immediately re- 

 ferred to the external air. When your head is immersed in water, 

 then the auditory sensations are not projected externally, but seem 

 to arise in the ear. 



Auditory Judgment. The auditory sensations inform us of the 

 nature, distance, and relative situation of bodies. The judgments 

 draw their exactness from associations established in previous ex- 

 periences between those of hearing and the other senses. When we 

 hear a particular instrument, the sensation we experience calls up a 

 picture of all its qualities which, from our past experience, we know 

 belong to that instrument. The appreciation of the distance of a 



Fig. 318. Semi-circular Canals on Right Side Destroyed, Com- 

 mencing rotation of head of pigeon about five days after the operation. 

 (After EWALD.) (From Tigerstedt's "Human Physiology/' copyright, 

 1906, by D. Appleton and Company.) 



body by its sound results from thousands of experiences between audi- 

 tory impressions of that body and the visual impressions. The auricle 

 has an important part in the determination of the direction of 

 sounds, causing an inequality of impressions which strike the two 

 ears. 



Semicircular Canals. The semicircular canals are, through the 

 vestibular nerve and the cerebellum, the most important agents in 

 the preservation of equilibrium. When in a pigeon the horizontal 

 canals are divided, the head moves from left to right and from right 

 to left, with nystagmus and a tendency to revolve on its vertical axis. 

 When the inferior vertical or posterior canals are divided, the head 

 oscillates from front to rear ; the animal has a tendency to fall back- 

 ward. A section of the superior vertical canal causes the head to 



