7 i8 



THE SENSES 



A radial layer of muscle, figure 445, is knitted into the base of the cornea, on 

 the one hand, and extends back into the choroid, on the other. Thick bun- 

 dles of the circular fibers are also present in this mass of muscle. From the 

 ciliary processes extending over the lens is the iris. It is a sheet of connec- 

 tive tissue and muscle lined with epithelium and highly pigmented. 



In the middle anterior portion of the iris is a round aperture, the pupil. 

 The muscle fibers are of the unstriated muscle type and are arranged in 

 two layers one circularly and the other radially. Contractions of the cir- 

 cular muscles of the iris produce constriction of the pupil, while contrac- 

 tions of the radial fibers produce dilatation. Both the ciliary apparatus 

 and the iris are supplied with motor nerves. 



anterior ciliary arteries and veins 



greater arterial circle 

 angle of the Iris 



canal of schlemm 



ciliary muscle 



limbus corneae 



anterior chamber 



epithelium 



anterior limiting! 

 membrane f 



posterior chamber 

 epithelium of lens 



capsule of lens 



posterior limiting membrane 



stroraa of Iris 

 posterior surface of lite 

 sphincter of pupil 



FIG. 445. Meridional Section of a Portion of the Anterior Part of the Eyeball. (Toldt.) 



Fibers of the third cranial nerve are distributed to the ciliary muscles, 

 apparently to both radial and circular muscles, and when these nerves are 

 stimulating the resulting contractions of the muscles tend to remove the ten- 

 sion from the capsule of the lens. These nerve fibers pass through the ciliary 

 ganglion where they form synapses with the ganglionic cells. Motor fibers 

 from the third cranial nerve also supply the circular muscle of the iris, which 

 produces constriction of the pupil. The radial muscles of the iris are 



