Chap. XIX.] ORGANS OF SPECIAL SENSE. 233 



the brain, and it is the function of the semicircular canals 

 to give us a knowledge of the position of the head when at 

 rest. The intensity and direction of the pressure of the oto- 

 liths upon the sensory hairs of the vestibule are also thought 

 to give us a like knowledge ; namely, the position of the head 

 when at rest. 



The sense of sight. — The eye is the special organ of the sense 

 of sight, and consists of the eyeball, or eye proper, and of acces- 

 sory protective appendages, such as the eyebrows, eyelids, lach- 

 rymal glands, etc. 



The eyeball is contained in a bony cavity, the orbit, which is 

 padded with fat and lined with a membranous capsule, — the 

 capsule of Tenon. This capsule is a serous sac, one layer of 

 which is attached to the posterior portion of the eyeball, while 

 the other lines the orbital cavity : in this way the eyeball is 

 isolated from surrounding structures, and free movement with- 

 out friction is insured. The orbit is shaped like a four-sided 

 pyramid ; the apex, directed backwards and inwards, is pierced 

 by a large opening — the optic foramen — through which pass 

 the nerves and blood-vessels distributed to the eyeball. The 

 base of the orbit, directed outwards and forwards, forms a strong 

 bony edge for protecting the eyeball from injury. 



The eyeball is spherical in shape, but its transverse diameter 

 is less than the antero-posterior, so that it projects anteriorly, 

 and looks as if a section of a smaller sphere had been engrafted 

 on the front of it. 



The eyeball is composed of three coats or tunics, and contains 

 three refracting media or humours. They are as follows : — 



Tunics. — 1. Sclerotic and cornea. 



2. Choroid, iris, and ciliary processes. 



3. Retina. 



Refracting media. — 1. Aqueous. 



2. Crystalline lens and capsule. 



3. Vitreous. 



The sclerotic (derived from the Greek word signifying hard) 

 covers the posterior five-sixths of the eyeball. It is composed 

 of a firm, unyielding, fibrous membrane, thicker behind than in 

 front, and serves to protect the delicate structures contained 

 within it. It is opaque, white and smooth externally, and 



