CHOROID. CILIARY LIGAMENT. 497 



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

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

 areolar tissue, and by 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 connected anteri- 

 orly with the iris, ciliary processes, and with the line of junction of 

 the cornea and sclerotic, by a dense white structure, the ciliary liga- 

 ment, which surrounds the circumference of the iris like a ring. 



The choroid membrane is composed of three layers: An external 

 or venous layer, which consists principally of veins arranged in a pecu- 

 liar manner: hence they have been named venae vorticose. The mark- 

 ing 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. The 

 middle or arterial layer (tunica Ruyschiana*) is formed principally 

 by the ramifications of minute arteries. It is reflected inwards at its 

 junction with the ciliary ligament, so as to form the ciliary processes. 

 The internal layer is a delicate membrane (membrana pigmenti) com- 

 posed of several laminae of nucleated hexagonal cells, which contain 

 the granules of pigmentum nigrum, ancfare 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 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,f from its dis- 

 coverer. 



was originally applied to the membrane of the foetus called chorion from the 

 Greek word X^^t domicilium, that membrane being, as it were, the abode or 

 receptacle of the foetus. Xo^wv comes from %tt>/>iu, to take or receive. Now 

 it so happens that the chorion in the ovum is a vascular membrane of peculiar 

 structure. Hence the term choroid, %o<>iav iftos, like the chorion, has been 

 used indiscriminately to signify vascular 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 1638, and was appointed professor of 

 Anatomy at Amsterdam in 1665. His whole life was employed in making in- 

 jected preparations, for which he is justly celebrated, and he died at the ad- 

 vanced age of ninety-three years. He came to the conclusion that the bofa 

 was entirely made up of vessels. 



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

 Canal in the Eye," was published in 17/8, in a Letter to the Professor of Ana- 

 tomy in Upsal. 



2 K 



