443 CILIARY LIGAMENT IRIS. 



Fig. 200.* *h e surface of the membrane pro- 



duced by these veins resembles so 

 many centres to which a number of 

 curved lines converge. It is this 

 layer which is connected with the 

 ciliary ligament. The middle or ar- 

 terial layer (tunica Ruyschiana\) is 

 formed principally by the ramifica- 

 tions of minute arteries. It is re- 

 flected inwards at its junction with 

 the ciliary ligament, so as to form 

 the ciliary processes. The internal 

 layer is a delicate membrane (mem- 

 brana pigmenti) composed of seve- 

 ral laminae of nucleated hexagonal 

 cells, which contain the granules of pigmentum nigrum, and are arranged 

 so as to resemble a tesselated pavement. 



In animals, the pigmentum nigrum, on the posterior wall of the eyeball, 

 is replaced by a layer of considerable extent, and of metallic brilliancy) 

 called the tapetum. 



The Ciliary ligament, or circle, is the bond of union between the ex- 

 ternal and middle tunics of the eyeball, and serves to connect the cornea 

 and sclerotic, at their line of junction, with the iris and external layer of 

 the choroid. It is also the point to which the ciliary nerves and vessels 

 proceed previously to their distribution, and it receives the anterior ciliary 

 arteries through the anterior margin of the sclerotic. A minute vascular 

 canal is situated within the ciliary ligament, called the ciliary canal, or the 

 canal of Fontana,J from its discoverer. 



The Ins (iris, a rainbow,) is so named from its variety of colour in dif- 

 ferent individuals : it forms a septum between the anterior and posterior 

 chambers of the eye, and is pierced somewhat t the nasal side of its centre 

 by a circular opening, which is called the pupil. By its periphery it is 

 connected with the ciliary ligament, and by its inner circumference forms 

 the margin of the pupil ; its anterior surface lks towards the cornea, and 

 the posterior towards the ciliary processes and lens. 



The iris is composed of two layers, an anterior or muscular, consisting 

 of radiating fibres which converge from the circumference towards the 

 centre, and have the power of dilating the pupil ; and circular, which sur- 

 round the pupil like a sphincter, and by their action produce contraction 



* A dissection of the eyeball, showing its second tunic, and the mode of distribution 

 of the venae vorticosae of the choroid. After Arnold. 1. Part of the sclerotic coat. 2. 

 The optic nerve. 3, 3. The choroid coat. 4. The ciliary ligament. 5. The iris. 6, 6. 

 The venae vorticosae. 7, 7. The trunks of the venae vorticosae at the point where they 

 have pierced the sclerotica. 8, 8. The posterior ciliary veins, which enter the eyeball 

 n company with the posterior ciliary arteries, by piercing the sclerotic at 9. 10. One 

 of the long ciliary nerves, accompanied by a long ciliary vein. 



j- Ruysch was born at the Hague in 1638, and was appointed professor of Anatomy 

 at Amsterdam in 1665. His whole life was employed in making injected preparations, 

 for which he is justly celebrated, and he died at the advanced age of ninety-three years. 

 He came to the conclusion that the body was entirely made up of vessels. 



t Felix Fontana, an anatomist of Tuscany. His " Description of a New Canal in the 

 Eve" was published in 1778, in a Letter to the Professor of Anatt my in Upsal. 



