632 THE SENSES 



The Lens. The lens is a special modification composed of highly 

 refractive material, situated just behind the iris. It is enclosed in a capsule 

 and supported in its place by the suspensory ligament, which is fused into 

 the capsule around its equator. The lens is a biconvex structure composed 

 of transparent fibers which are arranged in concentric layers. Its posterior 

 curvature is greater than the anterior, the radii being 6 and 10 mm. respectively. 



The Ciliary Apparatus and the Iris. These structures are a con- 

 tinuation and modification of the choroid coat in' the anterior portion of the 

 eye. Around the circumference of the cornea the choroid coat is consider- 

 ably thickened and folded in the modification known as the ciliary apparatus. 

 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 bundles 

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



FIG. 443- FlG - 444. 



FIG. 443. Ciliary Processes, as Seen from Behind, i, Posterior surface of the iris, with the 

 sphincter muscle of the pupil; 2, anterior part of the choroid coat; 3, one of the ciliary processes, 

 of which about seventy are represented. Xi. 



FIG. 444. Laminated Structure of the Crystalline Lens. The laminae are split up after hard- 

 ening in alcohol, i, The denser central part or nucleus; 2, the successive external layers. X4. 

 (Arnold.) 



processes, extending over the lens, is the iris. It is a sheet of connective tissue 

 and muscle lined with epithelium and highly pigmented. 



In the middle anterior portion is a round aperture, the pupil. The mus- 

 cle fibers are arranged circularly and radially and are of the unstriated muscle 

 type. Contractions of the circular muscles of the iris produce constriction 

 of the pupil, while contractions of the radial fibers produce dilatation. Both 

 the ciliary apparatus and the iris are supplied with motor nerves. 



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 tension 

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

 ganglion where they form a synapsis with ganglionic cells. Motor fibers 

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

 produce constriction of the pupil through the motor nerves by way of the 



