60 INNERVAT10N, [CHAP. XVII. 



double sensation is excited. Thus, in squinting, two impressions 

 are excited, unless, by long habit, one eye ceases to be adjusted and 

 employed, and gradually loses its excitability : but when two similar 

 objects are presented to the eyes of a squinting person, one carefully 

 in the axis of each, their images coincide and they are seen as one. 

 The double vision of drunkenness, and of certain cerebral affections, 

 is explicable partly on the same ground, but in such cases con- 

 siderable allowance must be made for the disordered state of the 

 sensorium. Again, if corresponding points of the two retinae are 

 pressed by the finger, a single luminosity is perceived ; but a double 

 one, if the points touched are non-symmetrical. Something similar 

 to this blending of two impressions in one sensation exists in the 

 sense of hearing, and, perhaps, also in taste and smell. 



The corresponding points of the two retinae are such as would be 

 in contact, if the two retinae were adapted to one another : the 

 upper and lower parts correspond with the upper and lower, and 

 the inner side of one with the outer side of the other. 



As we are entirely ignorant of the mode in which the mind 

 takes cognizance of a single impression on an organ of sense, we 

 cannot hope to understand how a single sensation can result from 

 a double impression. But it is most interesting to remark a 

 structural peculiarity in the course of the optic nerves, which 

 certainly allies itself with this wonderful part of their function. 

 The partial decussation in the chiasrna, or commissure, con- 

 nects each retina with both optic tracts, and with the cor- 

 responding portions of the cerebrum; and it is not improba- 

 ble, as Dr. Wollaston conceived, and Mr. Mayo has described, 

 that the right side of both retinae is continuous with the right 

 optic tract, and the left side of both with the left. This would 

 place each side of the central apparatus in connexion with its own 

 side of both the symmetrical images, and might be supposed to 

 favour their conception as one. Dr. Wollaston relates, that on 

 different occasions he lost the power of seeing one half of an object 

 to which he directed both eyes; and others have experienced 

 similar temporary attacks. Thus, Abernethy would humorously 

 affirm that he could sometimes see only his ne and thy, having lost 

 the other members of his name. Such phenomena are readily 

 explained by supposing the anatomical arrangement of the sides of 

 the retinae, with regard to the optic tracts, to be such as above 

 described, since any derangement of one optic tract would then 

 affect the same part of both optic images. Indeed, in Dr. Wollas- 

 ton' s own case, a tumour was found involving one of the optic 



