SECOND TUNIC. 447 



of the cornea proper," and of the lining membrane of the anterior chamber 

 of the eyeball The cornea elastica is remarkable for its perfect transpa- 

 rency, even when submitted for many days to the action of water or alco- 

 hol ; while the cornea proper is rendered opaque by the same immersion 

 To expose this membrane, Dr. Jacob suggests that the eye should be placed 

 in water for six or eight days, and, then, that all the opaque cornea should 

 be removed layer after layer. Another character of the cornea elastica is 

 its great elasticity, which causes it to roll up when divided or torn, in the 

 same manner as the capsule of the lens. The use of this layer, according 

 to Dr. Jacob, is to " preserve the requisite permanent correct curvature 

 of the flaccid cornea proper." 



The opacity of the cornea, produced by pressure on the globe, results 

 from the infiltration of fluid into the areolar tissue connecting its layers. 

 This appearance cannot be produced in a sound living eye. 



Dissection. The sclerotic and cornea are now to be dissected away 

 from the second tunic ; this, with care, may be easily performed, the only 

 connexions subsisting between them being at the circumference of the iris, 

 the entrance of the optic nerve, and the perforation of the ciliary nerves 

 and arteries. Pinch up a fold of the sclerotic near its anterior circumfer- 

 ence, and make a small opening into it, then raise the edge of the tunic, 

 and with a pair of fine scissors, having a probe point, divide the entire 

 circumference of the sclerotic, and cut it away bit by bit. Then separate 

 it from its attachment around the circumference of the iris by a gentle 

 pressure with the edge of. the knife. The dissection of the eye must be 

 conducted under water. 



In the course of this dissection the ciliary nerves and long ciliary arteries 

 will be seen passing forwards between the sclerotic and choroid to be dis- 

 tributed to the iris. 



SECOND TUNIC. The second tunic of the eyeball is formed by the 



ciliary ligament, and iris, the ciliary processes being an appendage 

 developed from its inner surface. 



The Choroid* is a vascular membrane of a rich chocolate-brown colour 

 upon its external surface, and tf a deep black c*Lur within. It is con- 

 nected 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 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 : an external or 

 venous layer, which consists principally of veins arranged in a peculiar 

 manner : hence they have been named vencs vorticosa. The marking on 



* The word choroid has been very much abused in anatomical language ; it was origi- 

 nally applied to the membrane of the fetus called chorion from the Greek word ^optoi-, 

 domicilium, that membrane being, as it were, the abode or receptacle of the fcetus 

 Xoptov comes from ^wps'w, to take or receive. Now it so happens that the chorion m 

 the ovum is a vascular membrane, of peculiar structure. Hence the term choroid, 

 opw>i/ ftSoj, 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 Cruveil- 

 liier in his work on Anatomy, vol. iii. p. 463, saying in a note, " Chorokle est synonym* 

 de vasculcuse." 



