212 HUMAN PHYSIOLOGY. 



Sapid substances, when in a state of solution, pass into the interior of the 

 taste-beakers, and come into contact, through the medium of the taste- 

 cells, with the terminal filaments of the gustatory nerves. 



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 posterior 

 nares, the latter communicating with the pharynx. They are lined by mucous 

 membrane, of which the only portion capable of receiving odorous impres- 

 sions is the part lining the upper one third of the fossae. 



The olfactory nerves, the olfactory bulb and tracts, unite the olfactory ep- 

 ithelium with the cortical areas of smell in the cerebrum. 



In animals which possess an acute sense of smell there is a corresponding 

 increase in the development of the olfactory bulbs. 



The essential conditions for the sense of smell are 



1. A special nerve center 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; this 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 is supported by a cushion of fat; it is protected from 

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

 situated as to permit of an extensive range of vision. The eyeball is loosely 

 held in position by a fibrous membrane, the capsule of Tenon, which is attached 

 on the one hand to the eyeball itself and on the other to the walls of the cavity. 

 Thus suspended, the eyeball is capable of being moved in any direction by 

 the contraction of the muscles attached to it. 



