

CILIARY LIGAMENT. 441 



colour upon its external surface, and of a deep black colour within. 

 It is connected to the sclerotic, externally, by an extremely fine cel- 

 lular tissue, and by the passage of nerves and vessels. Internally 

 it is in simple contact with the third tunic of the eye, the retina. It 

 is pierced posteriorly for the passage of the optic nerve, and is con- 

 nected anteriorly with the iris, ciliary processes, and with the line 

 of junction of the cornea and sclerotic, by a dense white structure, 

 the ciliary ligament, which surrounds the circumference of the iris 

 like a ring. 



The choroid membrane is composed of three layers : 1. An 

 external or venous, which consists principally of veins arranged in 

 a peculiar manner : hence they have been named vence vorticoses. 

 The marking upon the surface of the membrane produced 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. 2. The middle or arterial layer (tunica Ruyschiana*} is 

 formed principally by the ramifications of minute arteries, and 

 secretes upon its surface the pigmentum nigrum. It is reflected 

 inwards at its junction with the ciliary ligament, so as to form the 

 ciliary processes. 3. The internal layer is a delicate membrane 

 (membrana pigmenti) which presents a beautiful appearance beneath 

 the microscope ; it is composed of several lamina? 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, upon 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 

 external and middle tunics of the eye, and serves to connect the 

 cornea and sclerotic at their line of junction with the iris and ex- 

 ternal 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,f from 

 its discoverer. 



The Iris (iris, a rainbow) is so named from its variety of colour 

 in different individuals : ' it forms a septum between the anterior and 

 posterior chambers of the eye, and is pierced somewhat to the nasal 

 side of its centre by a circular opening, which is called the pupil. 



choroid, /f.fiov tUoc, like the chorion, has been used indiscriminately to signify vas- 

 cular structures, as in the choroid membrane of the eye, the choroid plexus, &c. and 

 we find Cruveilhier in his admirable work on Anatomy, vol. iii. p. 463, saying in a note, 

 " Choroide est synonyme de vasculeuse." 



* Ruysch was born at the Hague in 1 638, 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. He came to the conclusion that the body was en- 

 tirely made up of vessels. He died at the advanced age of ninety-three years. 



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

 the Eye," was published in 1778, in a letter to the Professor of Anatomy in Upsal. 



56 



