137 



perfectly satisfactory, if the anatomical facts are as the theory sup- 

 poses. The proof of the actual connexion of correspcniding points 

 of the retinae, with the same points in the corpora quadrigemina 

 or optic lobes (which physiological, still more than anatomical facts, 

 point out as the true origin of the optic nerves), is still defective; 

 but several facts stated on the subject by Wollaston, Mayo, Serres, 

 and others, tend to support the theory. 



In regard to correct vision by means of inverted and reverted 

 images on the retina of the eye, the author stated, and endeavoured 

 to shew by preparations, (what was 6rst suggested to him by Mr 

 Dick, veterinary surgeon,) that at least in the mammalia, and pro- 

 bably also lower in the scale of animals, the peculiar convoluted 

 course of the tractus optici, and the mode of their implantation into 

 the optic lobes, are such, that although the impressions made on the 

 retina by the different parts of an object are necessarily, by the 

 laws of optics, 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 optic lobes (i. e. on the 

 upper extremity of 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 de- 

 pends ; and therefore that the notions which we form of the rela- 

 tive position of the parts of objects by the senses of sight and touch 

 will naturally correspond. 



But although this seems sufficient to explain how the impressions 

 made by any object of sight on either optic lobe are in the same order 

 as those made on the sense of touch, yet in all animals which use 

 both eyes in looking at an object, and (by reason of the partial de- 

 cussation) both optic lobes in looking with one eye, another diffi- 

 culty presents itself. The impressions made on each lobe are in the 

 right order, but those made on the two lobes are transposed ; those 

 made by the left portion of the field of vision, even of a single eye, 

 falling on the right side of the retina, and being transferred to the 

 right optic lobe, and vice versa. Tiie error which would thence re- 

 suit, the author thinks, is rectified by means of another piece of 

 structure, the use of which has never been explained, viz. the decus- 

 sation at the pyramidal bodies, whereby, as is generally believed, 

 the whole sensation and voluntary motion of the left side of the 

 body are placed in connexion with the right side of the brain, and 

 vice versa. 



