THE SENSES. 743 



of a cube, A, B, fig. 441) be presented to corresponding parts of the two 

 retinae, as may be readily done by means of the stereoscope, the mind 

 will perceive not merely a single representation of the object, but a body 

 projecting in relief, the exact counterpart of that from which the draw- 

 ings were made. 



By transposing two stereoscopic pictures a reverse effect is produced ; 

 the elevated parts appear to be depressed, and vice versa. An instru- 

 ment contrived with this purpose is termed a pseudoscope. Viewed with 

 this instrument a bust appears as a hollow mask, and as may readily be 

 imagined the effect is most bewildering. 



There can be no doubt in order that the image of an object should fall 

 upon corresponding points in the two retinae, it is essential that the move- 

 ments of the eyes should be accurately co-ordinated, and the method of 

 this co-ordination is not so easily understood when examined carefully. 

 Thus, suppose the eyes be directed downward and to the left. On the left 

 side, the inferior rectus, the external rectus, and the superior oblique 

 would contract, and, on the right side the inferior rectus, internal rectus, 

 and superior oblique. In other words, a different set of muscles on 

 either side, and supplied to a certain extent by different nerves. There 

 must be some co-ordinating centre for these binocular movements. It is 

 thought that this centre is localized in the anterior corpus quadrigemi- 

 num, since stimulation of it causes conjugal lateral movement of the visual 

 axes to the opposite side, and stimulation at another spot produces move- 

 ments downward and inward. The posterior longitudinal bundle of fibres 

 described as found in the pons and crus, appears to be concerned in some 

 way with the simultaneous movement of the eyes; it appears to unite the 

 nuclei of the three nerves to the ocular muscles, the sixth, fourth, and 

 third. In it are said to be contained fibres from the sixth nerve of the 

 opposite side which go to the nucleus of the third nerve of the same side; 

 and this would serve to connect the nerve supply of the internal rectus 

 of one side, and the external rectus of the other side. It appears, how- 

 ever, that there is no evidence to assume that the fibres of the sixth 

 nerve decussate, but those of the fourth nerve do entirely, and those of 

 the third, partially. 



