of the Organs of Vision. 15 



rays of light, or pencils of light, which enter the eye ; they are 

 there refracted by the different humours ; and at last, by unit- 

 ing into a focus or point on the retina, form a perfect picture 

 of the objects from which they proceed. The impression 

 which is thus conveyed to the mind, through the medium of 

 the brain, constitutes the faculty which we call " vision." The 

 beautifully contrived instrument by which these changes and 

 impressions are effected and conveyed is called the ei/e. This 

 consists of a ball^ which is not a very regular sphere, though 

 it is usually called spherical ; the protuberance of the cornea 

 makes it bulge out in front. The eyeball is composed of a 

 certain number of coats or tunics^ and these serve as coverings 

 to the humours beneath. The tunics are : 1st, the sclerotica 

 {fg. 7. s), which forms nearly the whole of the outside covering 

 ^ „ of the orb or ball of 



-^=- the eye. It is, as 



its name indicates 

 (from sklerod, to 

 harden), hard and 

 firm. In the ante- 

 rior portion of the 

 orb is placed the 

 second tunic, called also, from its nature, cdrnea transpdrens (c) 

 (transparent horn). This is fitted into the first coat like a 

 glass into the frame of a clock. Immediately within the first 

 tunic, or sclerotica, is the choroid (c) [chorion^ from choreo, to 

 escape, and eidos, likeness. The chorion is a membrane which 

 is attached to the fcetus, and escapes with it at birth : from 

 its likeness to this membrane, the third tunic of the eye 

 derives its name.) The choroid is lined with a black liquid, 

 called the nigrum pigmentum (paint), and adheres to the internal 

 surface of the sclerotica ; but when it reaches the edge of the 

 latter, instead of being continued within the concave (or hol- 

 low) surface of the cornea, it hangs loosely down like a circu- 

 lar curtain, and forms the iris (i) (a rainbow) : and according 

 to the colour of the iris, in different individuals, the eye is said 

 to be blue, brown, grey, black, &c. The posterior surface of 

 the iris is called the uvea (so called because in beasts, which 

 the ancients chiefly dissected, it is found of a grape colour ; 

 uva meaning in Latin a grape). The large black hole in the 

 centre of the eye, surrounded by the iris, is called the pupa 

 (a puppet) or pupil (p). The blackness of this hole is owing 

 to the nigrum pigmentum which lines the choroid. The 

 French derive their term poupee, signifying a doll, from pupa ; 

 and the Greek anatomists termed the pupil kore, signifying a 

 diminutive person ; because, when we look at ourselves in a 



