348 HUMAN ANATOMY. 



vence verticosce, which emerge through the sclerotic midway be- 

 tween the optic nerve and the margin of the cornea. 



The internal layer is also composed of a fine capillary net- 

 work, derived from the short ciliary arteries, and continuous 

 in front with the vessels of the ciliary processes. It is called 

 the tunica Ruyschiana. 



The vitreous membrane lamina vitrea is a thin, structure- 

 less layer, separating the pigmentary layer of the retina from the 

 membrana Ruyschiana. 



The arteries are from posterior ciliary, short ciliary (prin- 

 cipally in the external layer), and recurrent branches of long 

 and anterior ciliary. 



The veins converge from the venae verticosae to form four or 

 five trunks, which pierce the sclerotic midway between the optic 

 nerve and corneal margin to join the cavernous sinus. 



The nerves are the long and short ciliary. 



The ciliary processes are a series of sixty to eighty pig- 

 mented vascular processes arranged circularly around the lens 

 behind the iris, and composed of the two internal layers of the 

 choroid folded inward. They are continuous in front with the 

 iris, and are connected behind with the suspensory ligament of 

 the lens. 



The structure is similar to that of the choroid. 



THE IRIS is a thin, circular curtain, suspended in the 

 aqueous humor, between the lens and cornea, and perforated by a 

 circular aperture the pupil. 



Its circumference is connected with the choroid, and also 

 by means of the ciliary ligament with the sclerotic and cornea. 

 The sclerotic contains in this position, near its junction with the 

 cornea, a circular canal (lymph canal, or venous sinus), the 

 sinus circularis iridis, or canal of Sclilemm. 



The circumference of the iris is also connected in front with 

 the cornea by ligamentum pectinatum iridis,, derived from the 

 membrane of Descemet. 



The structure is composed of four elements : 



1. A basement membrane and polyhedral cells, continuous with the 

 membrane of Descemet ; 



2. Stroma, consisting of bundles of fibrous tissue and cells; 



{Circular fibres, forming the sphincter of the 

 Ra^T/ing fibres, forming the dilator of the 

 pupil; 



4. Pigment, consisting of polyhedral or round pigment cells, dis- 

 tributed for the most part to the posterior surface, which, from its 

 deep-purple tint, has been called the uvea. 



