THE RESPIRATORY ORGANS 175 



how he breathes. The pernicious habit of breathing through 

 the mouth is easily overcome, either by a little care, or with 

 the aid of a surgeon; see page 177. 



Lachrymal Canals. Everyone has noticed that when tears 

 are running freely, they somehow get into the nose, whence 

 they may run out of the nostrils, but more often they pass 

 back to the throat and are swallowed (a child is seen swallow- 

 ing frequently when crying hard). Two tiny, ciliated canals 

 leave the inner corner of each eye and carry the tears to the 

 tear sac, which is located very near the eye in the tissues of 

 the nose. From each tear sac a canal, about three-quarters 

 of an inch long, passes down to open into the nose chamber 

 of that side where the secretion is ordinarily discharged. 



The Sense of Smell. The sense of smell is extremely deli- 

 cate. Whatever the something is which passes from an 

 object to the cells lining the nose, it must be exceedingly 

 minute, otherwise the object giving off the odor would entirely 

 waste away in a very short time. Perfectly dry substances, 

 such as balsam needles and sachet powders, give out fragrance 

 for years and yet do not lose appreciably in weight. If a 

 bottle of peppermint oil be opened for a few minutes, its odor 

 will fill a house, and yet if weighed in delicate scales, no ap- 

 preciable amount will be found to have disappeared from the 

 bottle. 



The upper passages in the nose, where the olfactory sense 

 is located, are separated from the brain by a very thin par- 

 tition, the ethmoid bone. This bone is perforated by numer- 

 ous short canals, through which olfactory nerves pass directly 

 from the front end of the brain. As soon as they enter the 

 nose they subdivide into numerous fine fibres which end 

 among the olfactory cells, both on the middle partition and on 

 the upper and middle turbinated surfaces; Fig. 96. 



The main passages through which air is drawn in ordinary 

 breathing lie in the lower part of the nose and go nearly 

 straight back from the nostrils to the openings into the 



