Double and Inverted Images on the Retince. 491 



true position of the objects whence they proceed ; yet, as the 

 optic nerves decussate completely, the vision of the left eye, and 

 therefore of objects on the left side of the body, will be dependent 

 on the right optic lobe, and vice versa. Here, it may be said, 

 there is an apparent cause of discord between tactual and visual 

 impressions ; yet there is no decussation at the Corpora pyrami- 

 dalia to compensate for it. 



I believe the true answer to this to be, that as the eyes of 

 these animals, in general, cannot be directed to the same point, 

 and as any object, accurately observed, is contemplated only by 

 one eye, so their attention is never fixed simultaneously on the 

 sensations of both eyes ; and when we attend to our own sensations, 

 when we produce artificial squinting, we shall see no difficulty in 

 this supposition. What makes it necessary, as I conceive, that the 

 decussation at the pyramids should exist, to preserve harmony 

 between the intimations of sight and of touch, is not the cir- 

 cumstance of the visual impressions from objects on the left side 

 of the body being made on the right optic lobe, but the circum- 

 stance of the impressions made on both optic lobes concurring in 

 producing one sensation, on which the attention is necessarily 

 fixed ; and it is where both optic lobes are found to be concerned 

 in vision, even by one eye, that this structure is therefore to be 

 expected. 



2. When it is said that the decussation at the pyramids trans- 

 fers the sensation and motion of the right side of the body to the 

 left side of the brain, the well known objection immediately pre- 

 sents itself, that the sensitive and motor nerves of the face arise 

 higher than that decussation ; and therefore that if this piece of 

 structure explains the harmony of impressions on the retinae, 

 with those on the body and limbs, it leaves unexplained the still 

 more remarkable fact, that impressions on the skin of the left side 

 of the face are felt to belong to the same side of the body, as im- 

 pressions on the right side of the retina, and therefore on the 

 right optic lobe. 



VOL. XIII. PART II. 3 R 



