610 HANDBOOK OF PHYSIOLOGY. 



an eye at its superior surface, so that the retina can be seen through the 

 vitreous humor, this reversed image of any bright object, such as the 

 windows of the room, may be perceived at the bottom of the eye. Or 

 still better, if the eye of any albino animal, such as a white rabbit, in 

 which the coats, from the absence of pigment, are transparent, is dis- 

 sected clean, and held with the cornea towards the window, a very dis- 

 tinct image of the window completely inverted is seen depicted on the 

 posterior translucent wall of the eye. Volkmann has also shown that a 

 similar experiment may be successfully performed in a living person 

 possessed of large, prominent eyes, and an unusually transparent scle- 

 rotic. 



An image formed at any point of the retina is referred to a point 

 outside the eye, lying on a straight line drawn from the point on the 

 retina outwards through the centre of the pupil. Thus an image on the 

 left side of the retina is referred by the mind to an object on the right 

 side of the eye, and vice versd. Thus all images on the retina are men- 

 tally, as it were, projected in front of the eye, and the objects are seen 

 erect though the image on the retina is reversed. Much needless con- 

 fusion and difficulty have been raised on this subject for want of re- 

 membering that when we are said to see an object, the mind is merely 

 conscious of the picture on the retina, and when it refers it to the ex- 

 ternal object, or " projects " it outside the eye, it necessarily reverses it 

 and sees the object as erect, though the retinal image is inverted. This is 

 further corroborated by the sense of touch. Thus an object whose pic- 

 ture falls on the left half of the retina is reached by the right hand, and 

 hence is said to lie on the right. Or, again, an object whose image is 

 formed on the upper part of the retina is readily touched by the feet, 

 and is therefore said to be in the lower part of the field, and so on. 



Hence it is, also, that no discordance arises between the sensations of 

 inverted vision and those of touch, which perceives everything in its 

 erect position; for the images of all objects, even of our own limbs, in 

 the retina, are equally inverted, and therefore maintain the same rela- 

 tive position. 



Even the image of our hand, while used in touch, is seen inverted. 

 The position in which we see objects, we call, therefore, the erect posi- 

 tion. A mere lateral inversion of our body in a mirror, where the right 

 hand occupies the left of the image, is indeed scarcely remarked; and 

 there is but little discordance between the sensations acquired by touch 

 in regulating our movements by the image in the mirror, and those of 

 sight, as, for example, in tying a knot in the cravat. There is some 

 want of harmony here, on account of the inversion being only lateral, 

 and not complete in all directions. 



The perception of the erect position of objects appears, therefore, to 

 be the result of an act of the mind. And this leads us to a considera- 



