384 STRUCTURE OF THE EYE. 



stance of optic nerve ; j?, ciliary circle of Zinn ; q, g, hyaloid mem- 

 brane ; r, capsular artery, lodged in hyaloid canal ; s, s, vitreous humor 

 and its cells ; t, crystalline and its capsule ; u, u, canal of Petit ; v, an- 

 terior chamber ; #, posterior chamber. 



To this general description of the conformation of the eye may be add- 

 ed a few remarks on each of its constituent parts. 



The sclerotic coat consists of white fibrous tissue, which, in addition 

 The sclerotic to the use before mentioned, affords the means of insertion of 

 coat. the muscular mechanism for moving the ball. It is thicker 



behind than in front, its relative thickness differing in different animals 

 according to the mechanical circumstances to which they have to be ex- 

 posed. In the whale, which has to resist the pressure of a deep sea, the 

 sclerotic is an inch thick. In some instances cartilage is included in it, 

 in others bone. Besides the aperture in front, into which the cornea is 

 let, there is another behind for the passage of the optic nerve. This 

 method of description, though very convenient, is, however, scarcely cor- 

 rect, if we consider the coats of the eye as arising from expansions of the 

 optic nerve, for then the sclerotic answers to the exterior investiture, and 

 the tubules of the nerve gain access to the interior of the eye without 

 passing through an aperture, properly speaking. The place at which 

 the nerve enters is not in the optical axis, but at a distance of about its 

 own diameter on the interior side. The aperture is smaller on the in- 

 side of the sclerotic than on the outside; thus it presents a conical shape. 

 It is not a single hole, but rather a collection of sieve-like openings, 

 through which the optic tubules pass. 



The cornea, which is let into the sclerotic in front, is of greater curva- 

 ture than the sclerotic. Its front and back faces are parallel. 



The cornea, m. , .. , ,, . -. , ., , . J . 



Though it seems to be pellucid as glass, it has a very intri- 

 cate construction, being composed of at least five separate layers ; the 

 innermost one, or cornea proper, consisting, it is said, of more than sixty 

 lamellae. 



The choroid coat is arranged, like the sclerotic, for the passage of the 

 Thechoroid: optic nerve. The iris is commonly described as a process 

 its arrange- O f ft. The choroid is a sheet of blood capillaries arranged 



ment for intro- . , . , , . , 



during and re- m two layers, an arterial and a venous, in such a way as to 

 moving blood. gj ve ^6 u tmost freedom of access for the arterial blood to 

 the retina within. The veins which remove this blood are placed in 

 curved forms, and are designated vasa vorticosa ; from the choroid also 

 the dark pigment is secreted. Those animals in which it is absent are 

 called albinos. Near to the iris the choroid merges in the ciliary liga- 

 ment, and gives forth the ciliary processes, being covered in front by the 

 ciliary muscle. 



Fig. 199 : a, a, section of sclerotic ; , exterior surface of the choroid, 



