60 A COURSE ON ZOOLOGY. 



dents which have resulted in its loss, the nose may be 

 replaced by an artificial nose without the olfactory organs 

 losing their sensibility. 



The nasal fossae receive the lachrymal secretion by a 

 duct coming from the internal angle of the orbit. This 

 maintains that condition of humidity of the pituitary 

 membrane necessary for its perception of odors. 



Hearing. The sense of hearing resides in the ear and 

 enables us to perceive sounds. 



The ear receives sounds as the nose receives odors, but 

 there is this difference, that while odors are probably 

 material particles emanating from odorous bodies, sounds 

 represent only the result of vibrations of a sounding body. 

 Sonorous vibrations are transmitted more or less perfectly 

 according to the elasticity of the medium interposed 

 between the vibrating body and the ear. They proceed 

 from one place to another like the waves produced by 

 a stone falling in water, and in this manner they pen- 

 etrate the ear, where they are received by the nerve 

 whose province is to transmit the impression to the 

 brain. 



Although the propagation of sound is quite rapid, it is 

 far from being instantaneous. Those who observe the 

 firing of a gun at a little distance notice the flash of light 

 before hearing the sound. In the air, sound is trans- 

 mitted with a velocity of about 1180 feet per second ; in 

 water, the velocity is four times as great, in iron ten 

 times, and in pine wood eighteen times as great. 



The ear is divided into three parts : the external ear 

 includes the concha, or auditory pavilion, as it is va- 

 riously called, and a canal hollowed out in the temporal 

 bone, lined with skin, and called the external auditory 

 canal. At the extreme interior of this canal is the 



