690 SPECIAL SENSES. 



as the cornea ; connected with the sclerotic by loose, connective tissue, in 

 which ramify blood-vessels and nerves, and presenting an external, vascular 

 layer and an internal, pigmentary layer, which latter gives its characteristic 

 dark-brown color. 



The ciliary processes ; peculiar folds of the choroid, which form its ante- 

 rior 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 shown by the dotted lines in the figure. 



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

 pied by the aqueous humor. The anterior chamber is much the larger. 

 The iris, in its central portion surrounding the pupil, is in contact 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 ex- 

 tending to about ^ of an inch (1*7 mm.) 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 situated next the pigmentary layer, which is 

 external. Internal to the layer of rods and cones, are the four granular lay- 

 ers ; 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 vitre- 

 ous humor, is the 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 anterior 

 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 be- 

 tween the folds of the ciliary processes, is called the zone of Zinn. The tri- 

 angular canal between the anterior and the posterior layers of the suspen- 

 sory ligament and surrounding the equator of the lens is called the canal of 

 Petit. 



The vitreous humor ; enveloped in the hyaloid membrane, which mem- 

 brane is continuous in front, with the suspensory ligament of the lens. 



REFRACTION IN THE EYE. 



In applying some of the elementary laws of refraction of light to the 

 transparent media of the eye, it is necessary to bear in mind certain general 

 facts with regard to vision, that have as yet been referred to either very briefly 

 or not at all. 



The eye is not a perfect optical instrument, looking at it from a purely 



