Double and Inverted Images on the Retince. 487 



these bodies (the nates), that the outer portions of the Tractus 

 pass over to the inner part of the nates, and the upper portions 

 of the Tractus pass down to the lower part of the nates.* 



In the lower classes of vertebrated animals, where the course 

 of the Tractus opticus on each side to the optic lobe, is shorter 

 and less winding, it is not so easy to ascertain whether the whole 

 of the fibres passing backwards from the nerve, and turning round 

 the crura cerebri, are inserted in the same way as in the mamma- 

 lia ; but that they are expanded over the optic lobes from before 

 backwards, is easily shewn, and if we can trust the representa- 

 tions of Serres, the mode of their implantation into the optic 

 lobes is quite in conformity with what we observe in the Mam- 

 malia, f 



Now, there is no such contortion or involution of the nervous 

 filaments of the fifth, or of any other nerve of the symmetrical sys- 

 tem, where it is implanted in the cerebro-spinal axis, and so con- 

 stituted a nerve of Touch ; and from this I think it clearly fol- 

 lows, that although the impressions made on the retina, by the 

 different parts of an object, are situated in regard to one another 

 in the inverse order of those made on the surface of the body, 

 yet the impressions made, through the retina and optic nerves, on 

 the cerebro-spinal axis, are in the same order, as those made through 

 the nerves of touch, on that central portion of the nervous system, 

 on which the sensibility of all nerves depends ; and therefore, 

 that the notions which we form of the relative position of the 

 parts of objects, by the senses of sight and of touch, will naturally 

 correspond. 



But another difficulty here presents itself. Although we un- 

 derstand, from what has been stated, in what manner the impres- 



* See Plate XVI. Fig. 2. of this volume. 



f See Anat. Comp. du Cerveau, Plates vi. vii. Figs. 149, 151, 159, 165, 170, 

 181, 188-89, and 19B ; and PI. XVI. Fig. 3. of this volume. 



