154 HUMAN PHYSIOLOGY. 



THE SENSE OF SMELL. 



The Sense of Smell is located in the mucous membrane lining the 

 upper part of the nasal cavity, in which the olfactory nerves are distributed. 



The Nasal Fossae are two cavities, irregular in shape, separated by 

 the vomer, the perpendicular plate of the ethmoid bone, and the triangular 

 cartilage. They open anteriorly and posteriorly by the anterior and pos- 

 terior nares, the latter communicating with the pharynx. They are lined 

 by mucous membrane, of which the only portion capable of receiving 

 odorous impressions is the part lining the upper one-third of the fossse. 



The Olfactory Nerves, arising by three roots from the posterior and 

 inferior surface of the anterior lobes, pass forward to the cribriform plate 

 of the ethmoid bone, where they each expand into an oblong body, the 

 olfactory bulb. From its under surface from fifteen to twenty filaments pass 

 downward through the foramina, to be distributed to the olfactory mucous 

 membrane, where they terminate in long, delicate, spindle-shaped cells, 

 the olfactory cells, situated between the ordinary epithelial cells. 



The olfactory bulbs are the centres in which odorous impressions are 

 perceived as sensations ; destruction of these bulbs being attended by an 

 abolition of the sense of smell. 



In animals which possess an acute sense of smell, there is a correspond- 

 ing increase in the development of the olfactory bulbs. 



The Essential Conditions for the sense of smell are, (i) a special 

 nerve centre capable of receiving impressions and transforming them into 

 odorous sensations. (2) Emanations from bodies which are in a gaseous 

 or vaporous condition. (3) The odorous emanations must be drawn freely 

 through the nasal fossae ; if the odor be very faint, a peculiar inspiratory 

 movement is made, by which the air is forcibly brought into contact with 

 the olfactory filaments. The secretions of the nasal fossae probably dissolve 

 the odorous particles. 



Various substances, as ammonia, horseradish, etc., excite the sensibility 

 of the mucous membrane, which must be distinguished from the perception 

 of true odors. 



THE SENSE OF SIGHT. 



The Eyeball. The eyeball or organ of vision is situated at the fore 

 part of the orbital cavity and supported by a cushion of fat; it is protected 

 from injury by the bony walls of the cavity, the lids and lashes, and is so 



