THE SENSE OF SIGHT. 237 



The essential conditions for the sense of smell are 

 i. A special nerve center capable of receiving impressions and trans- 

 forming 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 dis- 

 solve the odorous particles. 



Various substances, as ammonia, horseradish, etc., excite the sensi- 

 bility 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 mem- 

 brane, 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 direc- 

 tion by the contraction of the muscles attached to it. 



Structure. The eyeball is spheroid in shape and measures about 

 T 9 0- of an inch in its anteroposterior diameter, and a little less in its 

 transverse diameter. When viewed in profile, it is seen to consist 

 of the segments of two spheres, of which the posterior is the larger, 

 occupying five sixths, and the anterior the smaller, occupying one 

 sixth, of the ball. 



The eye is made up of several membranes, concentrically arranged, 

 within which are inclosed the refracting media essential to vision. 

 These membranes, enumerated from without inward, are 



1. The sclerotic and cornea. 



2. The choroid and iris. 



3. The retina. 



The refracting media are the aqueous humor, the crystalline lens, 

 and the vitreous humor. 



