488 Dr Arison on Single and Correct Vision, by means of 
sions, made on each of the optic lobes, by the images on the retinz, 
correspond with the real position of the parts of external objects, 
which these images represent ; yet, in the case of man, and of all 
other animals, in whom the partial decussation of the optic nerves 
exists, and in whom, if that form of structure has been rightly 
explained, both optic lobes are concerned in vision even by one eye, 
(the right lobe in the vision of the right division of the retina, 
and the left lobe in that of the left), only one portion of the field 
of vision, even of one eye, produces any impression on one optic 
lobe ; and the deft portion of the field of vision, being represented 
(by the laws of light) on the right division of the retina, makes 
its impression on the right optic lobe, while the right portion of 
the field of vision, represented on the left division of the retina, 
impresses the left optic lobe ; therefore, although the individual 
parts of each of these impressions are in the right order, yet the 
two impressions are transposed ; and both are necessarily, at one 
and the same moment, objects of attention to the mind. 
Now, if it be true, as is here supposed, that the impressions on 
the eye, by which we are informed of the relative position of ob- 
jects, harmonize with those made on the sense of touch, only 
because when transmitted to the optic lobes they are arranged 
in the real relative position of the objects exciting them, this 
transposition of the impressions made by two distinct portions of 
the field of vision, even of one eye, appears fitted to deceive us, 
and I believe would do so, were it not compensated by another 
piece of structure, the use of which has long puzzled physiologists, 
and which I do not remember to have seen connected by any one 
with the sensations of the eye, viz. the Decussation at the 
Pyramidal bodies ;* whereby, as is generally believed, the whole 
common sensation, and the whole voluntary motion, of the left 
half of the body, are put in connection with the right half of the 
brain, and those of the right half of the body with the left half of 
the brain. Therefore, while man, and all other animals, that 
have the power of looking directly forwards, see what is to their 
* Plate XVI. Fig 4. 
