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



and indeed in all animals the term partial decussation is the more 

 proper. In such animals, we should expect, from anatomical ob- 

 servation, what we find, from the effects of injury and disease, to 

 be the case, that the vision of each eye is more dependent on the 

 opposite optic lobe than on that on its own side. Partial decus- 

 sation, according to the theory, should exist, and, as far as obser- 

 vations have been made, I believe does exist, in all animals which 

 habitually bring their optic axes to bear on the same point, how- 

 ever distant ; but semi-decussation should exist only in those, in 

 which the natural direction of the axes is parallel. 



On the whole, it may be said, that there is very strong pre- 

 sumption, though not absolute certainty, in favour of the doctrine 

 of the dependence of single vision by two eyes on the partial de- 

 cussation of the optic nerves. 



II. The explanation, which seems to me satisfactory, of the 

 erect vision by inverted images, was first suggested to me by Mr 

 Dick, veterinary surgeon, and turns on the alleged fact, that the 

 course of the optic nerves and tractus optici is such, that impres- 

 sions on the upper part of the retina, are in fact impressions on 

 the lower part of the optic lobes, i. e. of the sensorium, and im- 

 pressions on the outer part of the former, are on the inner part 

 of the latter ; and vice versa. 



If this be so, it appears to me, after repeated consideration, 

 that it will furnish an explanation, and the only one of which the 

 subject admits, of the harmony or correspondence, which I believe 

 to exist from the first, between the intimations acquired by sight 

 and by touch, as to the relative position of objects or their parts, 

 notwithstanding that the impressions made by them on the ex- 

 ternal organs of sight and of touch are arranged inversely in re- 

 gard to one another. 



In regard to the nerves which are truly the organs of touch, 

 i. e. the posterior portions of the spinal nerves, and the larger 

 portion of the fifth cerebral, which is truly a spinal nerve, the ge- 



