THE IRIS, COl 



retina, where its meshes become larger, and join those of the ciliary 

 processes. 



On the inner sm-foce of the tunica Euyschiana is a structureless or 

 finely fibrillated transparent membrane, the membrane of Bri/ch, Avhich 

 lies next to the pigmentary layer of the retina, and anteriorly, in the 

 region of the ciliary processes, presents on its inner surface numerous 

 microscopic reticulating folds. 



The ciliary processes have the same structure as the rest of the 

 choroid ; but the capillary plexus of the vessels is less fine and has 

 meshes with chiefly a longitudinal direction ; and the ramified cells, 

 fewer in number, are devoid of pigment towards the free extremities of 

 the folds. 



The blood-vessels of the ciliary processes (fig. 413, d) are very numerous, 

 and are derived from the anterior ciliary, and from those of the fore 

 part of the choroidal membrane. Several small arterial branches enter 

 the outer part of each ciliary process, at first running parallel to each 

 other and communicating sparingly. As they enter the prominent 

 folded portion, the vessels become tortuous, subdivide minutely, and 

 inosculate frequently by cross branches. Finally they form short 

 arches or loops, and turn backwards to pour their contents into the 

 radicles of the veins. On the free border of the fold, one artery, 

 larger than the rest, extends the whole length of each cihary process, 

 and communicates through intervening vessels with a long venous trunk 

 which runs a similar course on the attached surface. 



Ciliary muscle. — At the anterior part of the choroid, between it and 

 the sclerotic, is a zone of plain muscular tissue, the ciliary muscle of 

 Bowman. It arises (fig. 403, 10) by a thin tendon from the forepart of the 

 sclerotic close to the cornea, and between the canal of Schlemm and the 

 sjDaces of Fontana, and its fibres, spreading out, are directed backwards 

 (fig. 403, K), to he inserted into the choroid opposite to the ciliary 

 processes, and partly further back, the fibres passing equatorially and 

 intercrossing so as to form peculiar stellate figures. A small portion 

 (the outermost) is sometimes inserted into the sclerotic (fig. 403, M). 

 These antero-posterior, or meridional and radiating fibres, pass at the 

 side next the iris into a ring of fibres (l), v.'hich have a circular course 

 around the insertion of the iris. This set forms the circular ciliary 

 muscle of H. Mliller. This circular muscle is much developed in 

 hypermetroi)ic eyes, but is atrophied, or may even, it is said, be absent 

 in myopic (Iwanoff). The ciliary muscle, at least its circular part, 

 was formerly described as the ciliary ligament. 



THE IRIS. 



The iris is the contractile and coloured membrane which is seen 

 behind the transparent cornea, and gives the tint to the eye. In its 

 centre it is perforated by an aperture — the pupil. 



By its circumferential border, which is nearly circular, the iris is 

 connected with the choroid, the cornea, and the ciliary muscles : the 

 free inner edge is the boundary of the pupil, and is constantly altering 

 its dimensions during life. The iris measures I an inch across, and, in 

 a state of rest, about ^th of an inch from the circumference to the pupil. 

 Its surfaces look forwards and backwards. The anterior, variously 

 coloured in different eyes, is marked by waved lines convergmg towards 



