410 TUNICA SCLEROTICA. 



over the whole internal surface of the eye, and is deficient in 

 the whole of the part occupied by the cornea. 



On the internal surface of the choroides is spread the pulpy 

 expansion of the optic nerve, called the Retina; the natural 

 consistence of which is not much more firm than mucus. 



Within these coats is a cavity that corresponds with the 

 figure of the sclerotica and cornea, but is divided by an incom- 

 plete membranous partition that separates the anterior part of 

 it, which is covered by the cornea, from the remainder. This 

 partition is called h-is ; and it has a circular vacuity in the 

 centre, directly opposite to the middle of the cornea, which is 

 denominated the Pupil. Rays of light, which penetrate the 

 transparent cornea, pass through the pupil into the posterior 

 part of the eye. 



The eye, thus formed, is filled with several transparent sub- 

 stances, called Humors. The greatest part of the cavity, 

 posterior to the iris, is occupied by the vitreous humor, which 

 is thus denominated, from its apparent resemblance to melted 

 glass. 



In front of the vitreous humor, and directly behind the 

 pupil, is a small body, with double convex surfaces, called the 

 Crystalline Lens or Crystalline Humor. 



In the space between the lens and cornea is a thin fluid, de- 

 nominated, from its consistency, the Aqueous Humor. 



Several of the parts, above enumerated, have exquisite deli- 

 cacy of structure, and require a very minute description. 



Of the Tunica Sclerotica. 



The Tunica Sclerotica, or external coat of the ball of the 

 eye, derives its name from a Greek word, which signifies to 

 make hard. It is composed of opaque white fibres, of great 

 firmness, which form a membrane of very close texture, that 

 supports the globular figure of the eye. It is thicker behind 

 than it is before ; but the expansion of the tendons of the recti 

 muscles gives it a' partial additional covering. — At its thickest 

 part, just at the outer side of the optic nerve, the sclerotica,' is 

 a line and a half thick. Immediately behind the insertion of 



