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. 

 8. 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 



