SPECIAL ANATOMY. 147 



superiorly and inferiorlj, so the part visible in the living 

 animal is markedly ovoid in shape, the largest end being 

 the inner. Around its outer circumferent margin we may 

 observe a semi-opaque ring, arcus senilis caused by the 

 presence of the ciliary ligament internally to this part. It 

 consists of two parts, cornea propria and cornea elastica ; the 

 former is anteriorly placed, and its structure may be divided 

 into several laminae ; the latter, posteriorly placed, consists 

 of a single lamina. The anterior surface is covered by the 

 transparent conjunctiva. Its posterior surface by the serous 

 membrane of the aqueous chamber. The sclerotic must 

 now be separated from the choroid or middle coat by laying 

 the eyeball on a flat surface, and cutting through the most 

 elevated portion of sclerotic with scissors, when the choroid, 

 which was separated by the weight of its contents will be 

 exposed. It is connected to the sclerotic by vessels and 

 nerves which form membrana fusca, distinguishable as a 

 few dark thread-like fibres running from one to the other. 

 The choroid is a dark brown membrane, on the surface of 

 which we see a number of light blue thread-like vessels 

 ramifying, forming the external layer of the choroid (venm 

 vorticoscE, so called from a fancied resemblance to a whirl- 

 pool) . The next layer mainly consists of arteries arranged 

 in festoons, of which the convexities are anteriorly situated, 

 tunica Uuyschiana. It rests upon a layer of polygonal pig- 

 ment cells, which constitutes the pigmentary layer, and in 

 which the rods and cones which form the external layer of 

 the retina, are imbedded. Towards the anterior part of 

 the sclerotic the choroid becomes reflected into the cavity 

 of the eyeball, to form the "setting" for the crystalline 

 lens ; and, as the diameter of the lens is less than that of 

 the chamber at this part, the choroid becomes puckered to 

 accommodate it to the size of the parts. The puckerings 

 are ciliary processes, the outer margin of which is termed 

 the ora serrata. The inner margin comes in contact with 

 the capsule of the lens to which it is attached, and between 

 the two is situated a triangular canal, the canal of Petit. 

 (Some uncertainty seems to exist as to the situation of this 

 canal. I have followed Huxley.) The ciliary processes 

 also contain some muscular fibres ; anteriorly they are 

 covered by the lining membrane of the aqueous chamber, 

 posteriorly by the retina. They are continuous with the 

 uvea, a layer of pigmentary matter which covers the pos- 



