THE OUTER AND INNER WORLDS 223 



that of the feeblest tone that can be heard by 

 the ear at the same distance ; one part of sul- 

 phate of quinine can be detected in 1,000.000 

 of water ; the odour of one part of bromine, 

 and even much less of iodoform may be de- 

 tected in 1,000,000 of air. Possibly the senses 

 of some animals are even more delicate. 



132. Each organ of sense has accessory 

 apparatus suitable to it. Thus the eyeball is 

 a camera for the purpose of forming, in accord- 

 ance with the laws of dioptrics, an image on 

 the retina. Vibrations of sound are conveyed 

 to the internal ear by a complicated conducting 

 mechanism of a drumhead, a chain of bones, 

 and reach a delicate organ in the cochlea, 

 known as the organ of Corti. By hair-like 

 processes in the semi- circular canals of the 

 inner ear, acted on by pressures of fluid in the 

 canals, varying according to the position 

 of the head, we appreciate the position of 

 the head in space, and we regulate the move- 

 ments of the body accordingly. In the 

 tongue and nose there are special epithelial 

 structures, such as the taste bodies and the 

 olfactory epithelium acted on by odoriferous 



