666 SPECIAL SENSES 



The ciliary processes ; folds of the choroid, which form its anterior 

 border and which embrace the folds of the suspensory ligament of the 

 lens. 



The ciliary muscle; situated just outside of the ciliary processes, 

 arising from the circular line of junction of the sclerotic with the cornea, 

 passing over the ciliary processes, and becoming continuous with the 

 fibrous tissue of the choroid. The action of this muscle is to tighten 

 the choroid over the vitreous humor and to relax the ciliary processes 

 and the suspensory ligament of the lens, when the lens, by virtue of its 

 elasticity, becomes more convex. This action is indicated by the dotted 

 lines in the figure. 



The iris ; dividing the space in front of the lens into two chambers 

 occupied by the aqueous humor. The anterior chamber is much the 

 larger. The iris, in its central portion surrounding the pupil, is in con- 

 tact with the lens. Its circumference is just in front of the line of origin 

 of the ciliary muscle. 



The retina ; a delicate transparent membrane, lining the choroid 

 and extending to about ^ of an inch (1.7 millimeters) behind the 

 ciliary processes, the anterior margin forming the ora serrata. The 

 optic nerve penetrates the retina a little internal to and below the 

 antero-posterior axis of the globe. The layer of rods and cones is situ- 

 ated next the pigmentary layer, which is external. Internal to the 

 layer of rods and cones and the outer limitary membrane, are the 

 molecular and nuclear layers ; next, the layer of nerve-cells ; next, 

 the expansion of the fibres of the optic nerve ; and next, in apposition 

 with the hyaloid membrane of the vitreous humor, is the inner limitary 

 membrane. 



The crystalline lens ; elastic, transparent, enveloped in its capsule 

 and surrounded by the suspensory ligament. 



The suspensory ligament ; the anterior layer connected with the an- 

 terior portion of the capsule of the lens, and the posterior, with the 

 posterior portion of the capsule. The folded portion of this ligament, 

 which is received between the folds of the ciliary processes, is the zone 

 of Zinn. The triangular canal between the anterior and the posterior 

 layers of the suspensory ligament and surrounding the equator of the 

 lens is the canal of Petit. 



The vitreous humor; enveloped in the hyaloid membrane, which 

 membrane is continuous in front with the suspensory ligament of the 

 lens. 



