452 ORGANS OF THE SENSES. 



The visual appreciation for perspective is a mental percep- 

 tion. The stereoscope produces a complete illusion of this 

 kind, doing for the mind what the latter would otherwise be 

 obliged to do for itself. In short, according to llelmholtz, in 

 the use of the stereoscope, we are conscious of two simulta- 

 neous sensations which are quite distinct from each other; the 

 blending of these two into a single image of the external 

 object is not caused by any pre-established mechanism for 

 the excitation of the organ of sense, but by the exercise of 

 the intellectual faculty. 



A satisfactory answer to questions of this kind is to be 

 found in the case of persons who are born blind, and 

 have been successfully operated upon. When they first 

 receive their sight, their visual impressions are the same as 

 ours, but the centre of visual perceptions has not received the 

 same education, in regard to its relation with other centres; 

 they lack what we have acquired. They usually imagine, 

 when they behold the outer world for the first time, that 

 every thing winch they perceive touches their eyes ; they have 

 neither the power of localizing nor of interpreting the im- 

 pressions made upon the retina. 1 



IV. Appendages of the eye. 



The appendages of the eye consist of the muscles by which 

 the eyeball is moved, and the lachrymal system which pro- 

 tects the front or exposed surface of this globe. 



Muscles of the Eye. If we consider how small a part of 

 the retina is really sensitive, we shall understand the impor- 

 tance of the movements of the globe of the eye or eyeball. 

 The eye may, indeed, be considered as a somewhat narrow 

 tube, which we can turn in any direction, for the purpose of 

 bringing the image of external objects into its deep median 

 part. These movements are effected by the muscles of the 

 globe of the eye. These muscles are, first, the recti muscles, 

 whose action we can readily understand. These are either 

 elevator or depressor muscles (superior and inferior recti 

 muscles), abductor or adductor muscles (the external and 

 internal recti muscles). The internal recti muscles are espe- 

 cially important because they serve to make the two visual 

 axes converge towards an object looked at with both eyes. 

 The combination of these muscles gives rise to every possible 



1 See the well-known history of the blind man of Cheselden, 

 in H. Taine, " De 1 'Intelligence," Vol.- II. Ch. 2. 



