662 



DISSECTION OF THE EYE. 



Fig. 232. 



movements of the iris are involuntary and regulate the admission of light 

 into the ball. 



The pigment cells are spread out in the stroma, and are disposed also on 

 both surfaces. In the stroma they are ramified and irregular, as in the 

 choroid (fig. 230, A ), and may contain yellow, brown, or very dark pig- 

 ment. On the front they are ovalish or rounded, but still ramified ; and 

 behind, where there is a thick layer (tivea), the cells are round without 

 outrunners, and are filled with granules. The color of the iris is dependent 

 upon the tint, and the position of the pigment. 



The arteries of the iris (fig. 232, B ) have a looped arrangement : they 

 are derived chiefly from the long and the anterior ciliary branches (d), 



but some come from the vessels of the 

 ciliary processes. On arriving at the 

 ciliary muscle the long ciliary arteries 

 form a circle (e) around the margin of 

 the iris ; from this loop other anasto- 

 motic branches are directed towards the 

 pupil, near which they join in a second 

 arterial circle (/). From the last circle 

 capillaries run to the pupil, and end in 

 veins. 



The veins resemble the arteries in 

 their arrangement in the iris, and ter- 

 minate in the veins of the choroidal 

 coat. 



The nerves of the iris (fig. 232, a) 

 divide into branches, which commu- 

 nicate, and extend towards the pupil; 

 they are without dark outline, and their 

 ending is not known. 



CILIARY VESSELS AND NERVES (fig. 

 231). The ciliary arteries are offsets 

 of the ophthalmic (p. 56), and supply 

 the choroid, the ciliary processes, and 

 the iris. They are classed into poste- 

 rior and anterior, and two of the first 

 set are named long ciliary ; but they 

 will not be seen without a special injec- 

 tion of the vessels of the eye. 



The posterior ciliary branches (^7) 

 pierce the sclerotic coat around and close 

 to the optic nerve, and running forwards on the choroid, enter its substance 

 at different points. 



Two of this set (long ciliary) are directed forwards, one on each side of 

 the eyeball, and form a circle around the iris in the ciHary muscle, as be- 

 fore explained. In the ball the outer one lies rather above, and the inner, 

 rather below the middle. 



The anterior ciliary arteries, five or six in number, are smaller than the 

 posterior, and arise at the front of the orbit from muscular branches (p. 

 56) ; they pierce the sclerotic coat about a line behind the cornea, supply 

 the ciliary processes, and join the circle of the long ciliary vessels. In 

 inflammation of the iris these vessels are enlarged, and offsets of them 

 form a ring around the cornea. 



DISTRIBUTION OF THE NBRVKS AND VES- 

 SELS OF THE IBIS. 



A. Half of the iris representing the nerves. 

 a. Nerves entering the membrane, and 



uniting in a plexus, b, within it (Kul- 

 liker). 



B. Half of the iris with apian of the ves- 



sels. 



d. Ciliary arteries. 

 . Arch of vessels at the onter edge of the 



iris. 



/. Inner circle of vessels in the iris. 

 g. Sphincter of the iris. 



