CILIARY VESSELS AND NERVES. 



797 



on-i.-ts of ^bundles of fibres which begin at the outer border of the 

 iris, and end internally in the sphincter. 



Action. Enlargement of the pupil is effected by shortening of the 

 radiating fibre-? : and diminution, by contraction of the circular ring. 

 The movements of the iris are involuntary, and regulate the admis- 

 sion of light into the hall. 



Tin' jiitjiit' ft i f the iris is partly interspersed in the substance of 

 the membrane, and partly collected into a thick layer on the pos- 

 ,erior aspect, the above-mentioned uvea, which is continuous with 

 the pigmentary stratum of the 

 retina. The colour of the iris 

 depends upon the nature and 

 quantity of the interspersed 

 pigment. 



The arteries of the iris (fig. 

 292, B) have a looped arrange- 

 ment ; they are derived chiefly 

 from the long and the anterior 

 ciliary branches (d), but some 

 ome from the vessels of the 

 ilian processes. On arriving 

 at the ciliary muscle, the long 

 and anterior ciliary arteries 

 form a circle (e) round the 

 margin of the iris ; from this 

 ring other anastomotic branches 



How they 

 act. 



Situation of 

 pigment. 



FIG. 292. DISTRIBUTION OF THE XERVE.S 

 AND VESSELS i..F THE IRIS. 



are directed towards the pupil, 

 near which they join in a 

 second arterial circle (/). From 

 the last circle capillaries run A. Half of the iris showing the nerves. 



to the edge of the pupil, and " Ne f rves . entei ; in ^ the embrane > 

 T . and uniting in a plexus. 



*. Within it. (Kolliker.) 

 B. Half of the iris with a plan of the Veins, 

 vessels. 



d. Ciliary arteries. 



e. Arch of vessels at the outer edge 

 of the iris. 



/. Inner circle of vessels in the iris. 

 g. Sphincter of the pupil. 



end in veins. 



The reins resemble the ar- 

 teries in their arrangements 

 in the iris, and terminate in 

 the veins of the choroid coat. 



The nerves of the iris are 

 the terminal branches of the 

 ciliary nerves ; they divide 



into branches which accompany the blood vessels, and communicate 

 with one another so as to form a plexus which gets gradually finer 

 towards the pupil (fig. 292, A). 



CILIARY VESSELS AND NERVES (tig. 289). The ciliary arteries 

 are offsets of the ophthalmic, and supply the choroid, the ciliary 

 processes, and the iris. They are classed into posterior and 

 anterior, and two of the first set are named long ciliary ; but 

 they will not be seen without a special injection of the vessels of 

 the eye. 



The posterior ciliary arteries (g) pierce the sclerotic coat around 

 and close to the optic nerve, and running forwards on the 



Nerves of 

 the iris. 



Arteries of 

 the middle 

 coat : 



posterior 

 ciliary, 



