784 SPECIAL SENSES. 



pensory ligament relaxed by the contraction of the ciliary muscle, and the lens, increased 

 in its convexity, pushing forward the iris. 



I., I. The iris ; dividing the space in front of the lens into two chambers occupied by 

 the aqueous humor : (A) The anterior chamber is much the larger. The iris, in its cen- 

 tral portion surrounding the pupil (P), is in contact with the lens. Its circumference is 

 just in front of the line of origin of the ciliary muscle. 



Ret., Ret. The retina ; an exceedingly delicate, transparent membrane, lining the 

 choroid and extending to about T V of an inch behind the ciliary processes, the anterior mar- 

 gin forming the ora serrata. O. The optic nerve penetrating the retina a little internal to 

 and below the antero-posterior axis. The layer of rods and cones is situated externally, 

 next the choroid. Internal to the layer of rods and cones, are the four granular 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 limitary 

 membrane. The layer of rods and cones is supposed to be the portion which receives 

 visual impressions, the rods and cones being connected with the nerve-cells, and through 

 them with the fibres of the optic nerve, by delicate filaments. The macula lutea and the 

 fovea centralis are exactly in the axis of distinct vision. 



C. The crystalline lens ; elastic, transparent, enveloped in its capsule and surrounded 

 by S. L., S. L., the suspensory ligament. 



S. L., S. L. 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 between the folds of the 

 ciliary processes, is called 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 called the canal of Petit. 



V. The vitreous humor ; enveloped in the structureless hyaloid membrane, which 

 membrane is continuous in front with the suspensory ligament of the lens. 



Refraction in the Eye. 



It is simply impossible to obtain a clear idea of the physiology of vision without hav- 

 ing carefully studied the physiological anatomy of the visual organs ; and, for this rea- 

 son, we have been as exact as possible and somewhat minute in our description of the 

 structure of the eye. If the student will carefully study the anatomy of the parts, a 

 very succinct statement of some of the well-established laws of refraction will render 

 the physiology so simple that it will follow almost without explanation. 



In applying the laws of the 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 by any means a perfect optical instrument, looking at it from a purely 

 physical point of view. This statement, however, should not be understood as implying 

 that the arrangement of the organs of vision is not such as to adapt them perfectly to the 

 functions which they have to perform in connection with the proper appreciation of 

 visual impressions. By physical tests, it can be demonstrated that the eye is not entirely 

 achromatic ; but, in ordinary vision, the dispersion of colors is not appreciated. There 

 is but a single point in the retina, the fovea centralis, where vision is absolutely distinct ; 

 and it is upon this point that images are made to fall when the eye is directed toward 

 any particular object. 



It is curious to note, however, that the refracting apparatus is not exactly centred, a 

 condition so essential to the satisfactory performance of our most perfect optical instru- 

 ments. For example, in a compound microscope or a telescope, the centres of the differ- 

 ent lenses entering into the construction of the instrument are all situated in a straight 

 line. Were the eye a perfect optical instrument, the line of vision would coincide ex- 



