228 SENSE OF SIGHT. 



Some, again, have advanced an opinion, that the decussation is par- 

 tial, and concerns only the internal filaments ; that the other filaments 

 pass directly on to half the corresponding eye ; so that one-half of each 

 eye is supplied by straight fibres proceeding directly from the root of 

 the same side; the other half by those resulting from the decussation 

 of the internal fibres. Messrs. Wollaston, 1 Berard, Pravaz, 2 Gall and 

 Spurzheim, Cuvier, Serres, Vicq-d' Azyr, Caldani, Ackermann, the bro- 

 thers Wenzel, G. R. Treviranus, J. Muller, Ruete, 3 and others, 4 em- 

 brace this opinion for the purpose of explaining the anomaly of vision 

 called hemiopia, in which only one-half the object is seen. MM. Cu- 

 vier, Serres, and Caldani assert, that they have noticed the above 

 arrangement in the nerves of the horse, when subjected to appropriate 

 maceration. More recently, Mr. H. Mayo 5 has stated that the optic 

 nerve consists in man of three tracts ; the innermost of which is wholly 

 commissural, connecting the two retinae anteriorly, and the optic gan- 

 glia posteriorly. The middle tract decussates, and is considered by him 

 to supply the part of the retina that lies on the inner side of each eye- 

 ball, between its anterior border and the entrance of the optic nerve. 

 The external tract, he affirms, does not decussate, but passes on to 

 supply the outer portion of the retina of the same side. Hence, the 

 right optic nerve, in Mr. Mayo's view, supplies the right side of each 

 eyeball ; and the left the left. Dr. Wollaston himself was affected with 

 hemiopia ; and, in his case, the loss of vision was sometimes on one 

 side, and sometimes on the other ; and he thought, that the phenomena 

 might be explained by partial decussation of the optic nerves ; but 

 Messrs. Solly 6 and Mayo have known instances of a like affection in- 

 volving alternately the centre and circumference of the retina, and 

 therefore not attributable to any such structural arrangement. 



These views are opposed, also, by the direct experiments of M. Ma- 

 gendie. 7 He divided, in a rabbit, the right optic nerve, behind the point 

 of decussation, or what has been called the chiasm of the nerves : 

 the sight of the left eye was destroyed. On cutting the left root, the 

 sight of the right eye was equally destroyed ; and on dividing the bond 

 of union, in another rabbit, by a longitudinal incision, made between 

 the nerves, vision was entirely abolished in both eyes ; a result, which, 

 as he properly remarks, proves not only the existence of decussation, 

 but also that it is total, and not partial as Wollaston had supposed. 

 Another experiment, which he instituted, led to a similar result. Fif- 

 teen days before examining a pigeon he destroyed one eye. The nerve 

 of the same side, as far as the chiasm, was wasted ; and, behind the 



Philosophical Transact., 1 824, p. 222. 



2 Archives Generates de Medecine, Mai, p. 59, Paris, 1825. 



3 Wagner's Handworterbuch der Physiologic, 16te Lieferung, s. 297, Braunschweig, 

 1847. 



4 Hildebrandt's Handbuch der Anatomie, von E. H. Weber, Band. iii. s. 438, Braunsch- 

 weig, 1832 ; Blumenbach, op. citat. ; Sir D. Brewster's Natural Magic, Amer. edit., p. 36, 

 New York, 1833 ; and. Pouillet, Elemens de Physique, iii. 338, Paris, 1832. 



6 London Medical Gazette, Nov. 5, 1841. 



6 The Human Brain, its Configuration, &e., p. 263, London, 1836; and Carpenter's Hu- 

 man Physiology, Amer. edit., p. 246, Philada., 1843. 



7 Precis, &c., edit. cit.. i. 64. 



