CHAP. XIX.] OBGANS OF SPECIAL SENSE. 223 



come directly from the brain through the cribriform plate, and 

 are essential to the sense of smell. Odorous particles in the air, 

 passing through the lower, wider air-passages, pass by diffusion 

 into the higher, narrower nasal chambers, and, falling on the 

 mucous membrane provided with olfactory nerve-endings, pro- 

 duce sensory impulses which, ascending to the brain, give rise 

 to the sensations of smell. 



If we wish to smell anything particularly well, we sniff the 

 air up into the higher nasal chambers, and thus bring the odor- 

 ous particles more closely into contact with the olfactory nerves. 



Each substance we smell causes its own particular sensation, 

 and we are not only able to recognize a multitude of distinct 

 odors, but also to distinguish individual odors in a mixed smell. 

 The sensation takes some time to develop after the contact of 

 the odorous stimulus, and may last a long time. When the 

 stimulus is repeated, the sensation very soon dies out, the 

 sensory terminal organs quickly becoming exhausted. Mental 

 associations cluster more strongly round sensations of smell 

 than round any other impressions we receive from without. A 

 whiff of fresh-mown grass ! What associations will it not con- 

 jure up for those happy mortals who spent their childish days 

 in country lanes and fields. 



The sense of hearing. The ear is the special organ of the 

 sense of hearing, and is made up of three portions, the exter- 

 nal ear, the middle ear or tympanum, and the internal ear or 

 labyrinth. 



. The external ear consists of an expanded portion, named 

 pinna or auricle, and the auditory canal or meatus. 



The auricle is composed of a thin plate of yellow fibro-car- 

 tilage, covered with skin, and joined to the surrounding parts 

 by ligaments and a few muscular fibres. It is very irregular 

 in shape, and appears to be an unnecessary appendage to the 

 organ of hearing, except that the central depression, the concha, 

 serves to some extent to collect sound-waves, and to conduct 

 them into the auditory canal. 



The auditory canal is a tubular passage, about an inch and a 

 quarter in length, leading from the concha to the drum-mem- 

 brane. It is slightly curved upon itself, so as to be higher in 

 the middle than at either end. It is lined by a prolongation of 

 the skin, which in the outer half of the canal is very thick and 



