828 



THE ORGANS OF SPECIAL SENSE. 



of perfectly transparent fluid will escape ; this is the aqueous humor. In the course of the 

 dissection the ciliary nerves may be seen lying in the loose cellular tissue between the choroid 

 and sclerotic or continued in delicate grooves on the inner surface of the latter membrane. 



II. The Choroid, Ciliary Body, and Iris. 



The Second Tunic of the Eye (tunica vasculosa oculi} is formed from behind 

 forward by the choroid, the ciliary body, and the iris. 



The choroid is the vascular and pigmentary tunic of the eyeball, investing the 

 posterior five-sixths of the globe, and extending as far forward as the ora serrata 

 of the retina; the ciliary body connects the choroid to the circumference of the 

 iris. The iris is the circular muscular septum, which hangs vertically behind the 

 cornea, presenting in its centre a large rounded aperture, the pupil. 



The Choroid is a thin, highly vascular membrane, of a dark-brown or chocolate 

 color, which invests the posterior five-sixths of the globe, and is pierced behind 

 by the optic nerve, and in this situation is firmly adherent to the sclerotic. It is 

 thicker behind than in front. Externally, it is loosely connected by the lamina 

 fusca with the inner surface of the sclerotic. Its inner surface is attached to the 

 retina. 



FIG. 441. The choroid and iris. (Enlarged.) 



Structure. The choroid consists mainly of a dense capillary plexus and of 

 small arteries and veins, carrying the blood to and returning it from this plexus. 

 On its external surface i. e., the surface next the sclerotic is a thin membrane, 

 the lamina superchoroidea, composed of delicate non-vascular lamellae, each lamella 

 consisting of a network of fine elastic fibres, among which are branched pigment- 

 cells. The spaces between the lamellae are lined by endothelium, and open freely 

 into the perichoroidal lymph-space, which, in its turn, communicates with the 

 perisclerotic space by the perforations in the sclerotic through which the vessels 

 and nerves are transmitted. 



Internal to this is the choroid proper, and, in consequence of the small arteries 

 and veins being arranged on the outer surface of the capillary network, it is 

 customary to describe this as consisting of two layers : the outermost, composed 

 of small arteries and veins, with pigment-cells interspersed between them, and 

 the inner, consisting of a capillary plexus. The external layer or lamina vasculosa 



