THE GLOBE OF THE EYE. 



931 



Fig. 503. 



fibres — numerous bundles near the sclerotic, and extending to the choroid, 

 forming the tensor choroideoe. 2. Radial fibres, next the meridional, radiating 

 towards the centre of the eye. 3. Circular or equatorial fibres, constituting the 

 sphincter or ciliary muscle of Miiller. 



The ciliary muscle originates on the inner wall of a minute circular canal — • 

 named the ciliary canal, canal of Schlemm, sinus 

 circularis iridis, circulus venosus orbicuU ciliaris, 

 or camd of Fontana, from its discoverer. It is 

 surmised to be a venous sinus, as it can always 

 be injected from the arteries.) 



Structure. — The choroid zone is composed 

 of four superposed layers : 1. The external is 

 formed by a network of connective 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 drooping 

 branches, to form the vasa vorticosa — Fig. 505, 

 2, 4 ; these are chiefly on the outer surface of 

 the layer, the arteries ramifying on the inner 

 surface.) The veins open into the ophthalmic 

 vein. 3. The third layer, or tunica Ruyschiana, 

 has for its basis an amorphous substance con- 

 taining a network of exceedingly fine capillaries 



(extending to the ciliary processes). 4. The interned layer is composed of hexagonal 

 cells, regularly placed one upon the other on the surface of a limitary membrane ; 

 the cells are provided with a nucleus, and contain pigment-granules which ex- 

 clusively occupy their anterior moiety. (On the choroid 

 this cell-formation is single, but on the iris and ciliary 

 processes there are several layers. A very delicate mem- 

 brane — membrane of Bruch, or vitreous membrane — has 

 been described as lining the inner surface of the choroid, 

 and retaining the pigment in its place ; this membrane 

 may be seen on the posterior surface of the iris, and it 

 probably prevents the pigment being removed by the 

 aqueous humour.) The use of the choroid membrane 

 is to convert the ocular globe into a veritable darkened 

 chamber, and to constitute for the retina a calefactory 

 apparatus. (The pigment absorbs the rays of light which 

 pass through the retina, and thus prevents their becoming 

 reflected and confusing the vision. The brilliant metallic- 

 coloured layer named the tapetum is more particularly 



observed in nocturnal animals, and especially in the Carnivora. By reflecting 

 the rays of light a second time through the retina, it probably enables the animal 

 to see better at night. It is the cause of the glare perceived in the eyes of 

 Cats and other creatures, in the dark.) 



The ciliary circle is a contractile body, being composed of unstriped musculai 



ANTERIOR SEGMENT OF A TRANSVERSE 

 SECTION OF THE GLOBE OF THE EVE 

 (human), SEEN FROM WITHIN. 



I, Divided edge of the three tunics — 

 sclerotic, choroid (the dark layer), 

 and retina ; 2, pupil ; 3, iris (the 

 uvea) ; 4, ciliary processes ; 5, den- 

 ticulated anterior border of the 

 retina. 



Fig. 504. 



CELLS FROM THE CHOROID 

 COAT. 



a, Pigmentary granules 

 concealing the nucleus ; 

 6, the nucleus distinct. 



