546 



THE NERVOUS SYSTEM. 



Margin of 



pupil. 



iris. It is this pigment which, as seen through the layers of the 

 iris anterior to it, gives the color to light eyes, while in those 

 having dark eyes there is, besides, pigment in the fibrous tissue of 

 the stroma and on the anterior surface of the iris. The albino 

 is characterized by the colorless iris, in which no pigment is 

 present. 



The arteries which supply the iris are branches of the long 

 and anterior ciliary, which together form the circulus iridis major 

 and minor, the former being an anastomotic ring at the outer 

 margin of the iris ; the latter, a similar one near the pupil. 



The nervous supply of the iris is derived from the ciliary 



ganglion and from the nasal 

 branch of the ophthalmic 

 division of the fifth nerve 

 through the long ciliary. 

 The branches from the cil- 

 iary ganglion contain fibers 

 of the third nerve, which 

 supply the circular muscular 

 fibers or sphincter pupillse, 

 and also sympathetic fibers, 

 which are distributed to the 

 radiating fibers or dilator pu- 

 pillae. 



Membrana Pupillaris. 

 The pupil during fetal life, 

 until about the seventh or 

 eighth month, is closed by a 

 membrane. At this time it 

 begins to be absorbed, and 

 the absorption is almost en- 

 tirely complete at birth. 



Ciliary Muscle (Fig. 323). 

 Like the muscular fibers of 

 the iris, this muscle is also 

 of the unstriped variety. Its 

 width is about 0.3 cm., and it 

 consists of two parts, a ra- 

 diating and a circular. The radiating or radial fibers are the more 

 numerous, and have their origin at the junction of the cornea and 

 sclerotic, sclerocorneal junction, and passing backward are attached 

 to the choroid opposite the ciliary processes. Waldeyer states that 

 one bundle is attached to the sclerotic. Internal to these are the 

 circular fibers, running around the attachment of the iris, called 

 circular ciliary muscle, ring muscle of Mutter, and formerly ciliary 

 ligament. These fibers are said to be most marked in hyperme- 

 tropic eyes. 



Choroid. 



FIG. 325. Injected blood-vessels of the 

 human choroid and iris; X 7 (Bohm and 

 Davidoff). 



