THE EYE. 



821 



(or ligament) " and the " ciliary body." The ciliary circle, ligament, or 

 muscle (annulus albidus) varies in width from one to two millimetres; its 

 external face adheres closely to the sclerotic, and its internal is confounded 

 with the ciliary body; the posterior border is continuous with the 

 choroid zone, near the canal of Fontana (ciliary canal). The anterior 

 border gives attachment to the greater circumference of the iris. Its 

 structure and uses will be referred to hereafter. 



The ciliary body (corpus ciliare) forms a kind of zone or ring, wider than 

 the ciliary ligament, and consequently overlaps the latter before and 

 behind. It extends, on one side, on the inner face of the choroid, and on 

 the other, on the posterior face of the iris. When the cornea and sclerotic 

 are removed so as to expose the ciliary ligament, this zone is not seen ; and 

 to discover it, it is necessary to excise all the posterior part of the shell of 

 the eye by a circular incision, and evacuate the vitreous humour. We then 

 observe, around the crystalline lens, a wide, black circle, forming very 

 regular radiating folds (ciliary processes) projecting inwards by their inner 

 extremities, and appearing in the posterior chamber of the eye, after cutting 

 away the iris ; all abut by these extremities on the circumference of the 

 lens, which they do not quite reach, although the latter is sustained by, and 

 " set " in, the middle of the ligament. 



These radiating folds (Fig. 384, 4) are from 110 to 120 in number in the 

 Horse, and are constituted by little parallel leaves, wider at their inner than 

 their outer extremity ; the furrows that separate them posteriorly are 

 partly concealed by the prolongation of the retina that constitutes the 

 zonula of Zinn. The coronet formed by the ciliary processes is usually 

 asymmetrical. 



(Between the sclerotic, the cornea, and the ciliary ligament, exists a 

 minute circular canal the ciliary canal, 

 canal of Schlemm, sinus circularis iridis, 

 circulus venosus orbiculi ciliaris, or canal of 

 Fontana, from its discoverer. It is sur- 

 mised to be a venous sinus, as it can 

 always be injected from the arteries.) 



STRUCTURE. The* choroid zone is com- 

 posed of four superposed layers: 1, The 

 external is formed by a network of con- 

 nective elastic fibres, among which are 

 disseminated a great number of pigment 

 cells ; 2, The second layer is constituted by 

 a network of large arteries and veins 

 the posterior ciliary and a plexus of 

 nerves (ciliary) accompanied by ganglia and 

 some (stellate) pigment cells. (The veins 

 are arranged with great regularity in droop- 

 ing branches, to form the vasa vorticosa 

 (Fig. 386, 2, 4) ; these are chiefly on the 



outer surface of the layer, the arteries 1? Divided ed of the three tuni 

 ramifying on the inner surface.) 3, The sclerotic, choroid (the dark layer), 

 third layer, or tunica Ruyschiana, has for 

 its basis an amorphous substance containing 

 a network of exceedingly fine capillaries 

 ^extending to the ciliary processes) ; 4, The 

 internal layer is composed of hexagonal cells, regularly placed one upon the 



Fig. 384. 



ANTERIOR SEGMENT OF A TRANSVERSE 



SECTION OF THE GLOBE OF THE EYE 

 (HUMAN), SEEN FROM WITHIN. 



and retina ; 2, Pupil ; 3, Iris, the 

 uvea ; 4, Ciliary processes ; 5, Den- 

 ticulated anterior border of the 

 retina. 





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