178 THE EYI5. 



part of the inner side of the orbit, it takes its course through 

 a fibro-cartilaginous pulley (d), lined by a delicate synovial 

 membrane to prevent friction, and its tendon leads outwards and 

 backwards, and expands to its insertion into the globe, which, 

 by its action, it rolls on its antero-posterior axis. Its opponent 

 muscle, the inferior-oblique, is placed below, and is short, and 

 arising from the bone of the inner and lower front corner of the 

 orbit. The action of these last two muscles is required for the 

 correct viewing of an object when the head is moved laterally, 

 as from shoulder to shoulder. 



Passing on to the eyeball itself, the globe (Fig. 3) is composed 

 of three investing tunics — 



1. Sclerotic and Cornea ; 



2. Choroid, Iris, and Ciliary processes ; 



3. Retina ; 



And three fluid and solid refracting media — ■ 



Aqueous ; 



Crystalline (Lens) and Capsule ; 



Vitreous. 



The Sclerotic, Fig. 3 (^), as its name implies, is extremely 

 dense and hard, being composed of firm, unyielding, fibrous 

 tissue, mixed with some elastic fibres. It serves to maintain the 

 form of the globe, and is much thicker behind than in front, the 

 principle strain from pressure applied in front falling behind, and 

 this is the main reason of the enormous relative thickening of the 

 sclerotic in the eye of the whale. Besides, in front, the tendons, 

 being closely applied, strengthen and protect the cornea, and 

 posteriously, the extremely sensitive portions of the retina 

 require protection from any disturbance. 



The Cornea, Fig. 3 (/), is the projecting, transparent part of 

 the external tissue of the eyeball, and forms the anterior sixth of 

 the globe. Its degree of curvature varies in different individuals, 

 and is more prominent in youth than in advanced life. The 

 cornea is dense and uniform throughout. It is not circular, even 

 in the human eye. According to Cuvier, the vertical chord of 

 the cornea is '46 inch, while the horizontal chord is "49. 



The curvature of the cornea is different in the eyes of other 



