351 



OF THE HIGHER ORGANS OF SENSE 



I.— OF THE ORGAN OF VISION. 



§ 224. 



The visual organ consists of the eye-ball, or the proper sen- 

 sitive apparatus, and the accessory parts, some for its protection, 

 some for its movement : viz. the eye-lids, the ocular muscles and 

 the lachrymal organs. The eye-ball itself is a very complex 

 organ, into the constitution of which nearly all the tissues of 

 the body enter. It is composed essentially of three tunics : 

 a fibrous — the sclerotica and cornea, a vascular — the chorioidea 

 and iris, and a nervous; and of two internal refractive media — 

 the vitreous humor and the crystalline lens. 



A. OF THE EYE-BALL. 



§ 225. 



Fibrous tunic of the Eye. — The external envelope of the 

 eye-ball is formed by a tough, fibrous membrane, composed 

 chiefly of connective tissue, which, to outward appearance, is 

 divisible into a smaller, anterior, transparent portion — the 

 cornea; and a larger, opaque, posterior part — the sclerotic; 

 but, as shown by its development and more intimate structure, 

 is to be regarded as a membrane continuous throughout. 



The sclerotica, also termed the tunica albuginea, is a white, 

 very tough and strong, fibrous membrane, which gradually 

 diminishes in thickness as it advances forwards from the pos- 

 terior part of the eye, where it is directly connected with the 

 sheath of the optic nerve, although it is again strengthened, 

 in front, by the expanded tendons of the recti muscles, with 

 which it is blended, afterwards becoming continuous with the 

 cornea. When boiled, it affords common gelatin, and it con- 

 sists of true connective tissue, the fibrils of which are verv 



