Double and Inverted Images on the Retine. 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 retinz, 
with those on the body and limbs, it leaves unexplained the still 
more remarkable fact, that impressions on the skin of the /eft 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. 3R 
