488 Dr Alison on Single and Correct Vision, by means of 



sions, made on each of the optic lobes, by the images on the retinae, 

 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 left 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. 



