458 SPECIAL PHYSIOLOGY. 



by the vertical and horizontal lines appear to be right angles; but, 

 when looked at with the right eye only, they appear to deviate from 

 right angles, and vice versd with the opposite figure. Two such figures, 

 however, combine stereoscopically, and then make an impression on 

 the mind, of a figure composed of perfect squares. In the act of ad- 

 justment necessary for this, not only must the eyes be made to con- 

 verge, by being rotated on their vertical axes by the internal recti 

 muscles, but the oblique muscles must slightly rotate the balls of the 

 eyes on their antero-posterior, or visual, axes (see p. 421). 



That the convergence of both optic axes at a given object, is essen- 

 tial for single vision, is proved by the following curious experiment. 

 If two small balls be placed near the further ends of two tubes, it will 

 be found, on looking through the tubes with both eyes, that when the 

 balls are brought nearer to the eyes, and these latter are made to con- 

 verge, the two eyes receive the impression of a single ball. Each eye 

 realizes its own image of one of the two balls; but the mind is conscious 

 that each image occupies a certain point in space, and these points 

 being felt, through the muscular sense of the convergence, to coincide, 

 we judge that one body only can occupy the same point of space at 

 one time. From this experiment, it would seem, that single vision 

 with the two eyes is not a simple sensation, but the result of experi- 

 ence, or of judgment, from facts presented to the visual and muscular 

 senses. 



Such being the conditions as to the position of the two eyeballs, 

 necessary to single vision, various theories have been advanced, to 

 account for the actual combination by the mind, of the two images. 

 ' By some, this is thought to be dependent on a fixed structural condi- 

 tion, such as a peculiar arrangement of the fibres of the optic nerves, 

 which decussate at the optic commissure, and are supposed thus to 

 bring certain parts of the two retinae into identical or corresponding 

 relations, as regards the sensorium. The right halves of the two re- 

 tinae are supposed to be connected, by the fibres of the two optic 

 nerves, with the right optic sensorial nervous centres, and the left 

 halves of the retinae, in like manner, with the left optic centres, this 

 arrangement being effected by a supposed semi-decussation of the 

 fibres of each optic nerve. If such an arrangement existed, each 

 optic sensorial centre would obviously receive impressions belonging 

 only to the same side of the field of vision. It is probable, however, 

 that the decussation of the fibres of the optic nerves is not thus par- 

 tial, but almost, if not quite, complete; for amaurosis of one eye, or 

 section, in animals, of one optic nerve, is followed by withering of the 

 opposite optic tract only. Moreover, in any case, the sensorial im- 

 pressions, being received by separate bilateral optic nervous centres, 

 are, of course, themselves bilateral or double, and the necessity of ex- 

 plaining their subsequent combination by the mind, still remains. 



The actual combination of the two impressions, like the combination 

 of those of hearing, smell, and taste, must, however, be the result of 

 some mental operation, either intuitive, experimental, or rational. 

 There is no doubt that the convergence of the two eyes to a fixed 

 point in the object, is a necessary condition of seeing it single ; and 



