ANATOMY OF THE EYE 



105 



The second coat is formed by the choroid ciliary processes and iris 

 (tig. 240, E, m). 



The third coat is the retina, or expansion of the optic nerve (fig. 240, f). 

 The three humours are the aqueous humour (fig. 240, k), the crystalline 

 lens (fig. 240, j), and the vitreous humour (fig. 240, h). 



The outermost tunic is a dense, tough, and unyielding membrane, 

 the inner four-fifths of which is opaque, and named the sclerotic or white 

 of the eye, whilst the outer or front fifth is transparent, and is named the 

 cornea or glass of the eye. The sclerotic is thick behind, where it presents 

 a small opening for the entrance of the optic nerve (fig. 248, 1), and thinner 

 in front, where it becomes continuous with the cornea. The fore part of 

 the sclerotic, or that which forms 

 the white of the eye, is covered 

 with a delicate mucous membrane, 

 named the conjunctiva, which is 

 continued over the cornea as a 

 transparent membrane composed 

 of eight or ten layers of cells. 

 These cells, if injured, can I to 

 thrown off and renewed, so that 

 the transparency of the cornea is 

 not impaired after slight lesions. 

 Different as the aspect of the 

 cornea is from the white or sclerotic 

 portion of the membrane, the ma- 

 terial out of which each is com- 

 posed — connective tissue— is the 

 same, only that in the sclerotic the fibres are irregularly arranged, whilst 

 in the cornea they are disposed in layers or lamellae one upon another, 

 with' many intervening branching cells which enable the nutrient fluid 

 or plasma of the blood to penetrate and nourish the tissue. The cornea 

 is supplied by the fifth pair of nerves, which confer upon it the exquisite 

 sensitiveness of the surface. 



The Choroid Tunic is composed of a close net-work of blood-vessels, 

 the outer layer of which is formed by the large veins of the eye known as 

 venae vorticosse (fig. 241, 2). The inner layer is formed by the delicate 

 capillary vessels disposed in loops, and is in contact with the retina. At 

 the back part of the choroid there is a circular opening, like that in the 

 sclerotic, for the passage of the optic nerve to the retina; in front the 

 choroid is folded into a large number of plaits named the ciliary processes 

 (fig. 242, c), which project into the interior of the eye behind the iris. 



Fig. 241.— The Choroid Tunic 

 1, Cut Surface of Sclerotic. 2, Veins of the Choroid 

 (Venai Vorticosce). 3, C'iliary Nerves and Arteries. 

 4, Ciliary Ligament. 5, Iris. 6, Pupil. 



