228 ANATOMY FOE NUKSES. [CHAP. XIX. 



the neurones are stimulated directly, and not through the inter- 

 vention of modified epithelial cells. The axones of these cells 

 unite to form numerous bundles of fibres which pass upward 

 through the cribriform plate of the ethmoid bone and terminate 

 in the olfactory bulb of the brain. 



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, produce sensory impulses which, ascend- 

 ing to the brain, give rise to the sensation 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 

 odours, but also to distinguish individual odours in a mixed 

 smell. The sensation takes some time to develop after the con- 

 tact 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 ear. The ear is the special organ of the sense of hear- 

 ing, and is made up of three portions, the external 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 (32 mm.) in length, leading from the concha to the 

 drum-membrane. It is slightly curved upon itself, so as to 



