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nerves of the orbit of the eye. 
But it has been observed, in the First Part of this Paper, 
that the shutting of the eyelids is not the only part of this act 
of preservation, and that the motions of the eyelids are at- 
tended with a rolling of the eyeball. How is this relation 
between the eyelids and eyeball established ? This leads to an 
examination of the fourth nerve. 
The fourth nerve. 
This is a fine nerve, which takes its origin from the brain, 
at a part remote from all the other nerves which run into 
the orbit. It threads the intricacies of the other nerves with- 
out touching them, and is entirely given to one muscle, the 
superior oblique. We may observe too, that this singularity 
prevails in all animals. What office can this nerve have in 
reference to this one muscle ? Why is it’s root, or source, 
different from the other nerves, from the nerve of vision, the 
nerve of common sensibility, and the nerve of voluntary 
motion ? We now reflect, with increased interest, on the 
offices of the oblique muscles of the eye, observing that they 
perform an insensible rolling of the eyeball, and hold it 
in a state of suspension between them. We have seen that 
the effect of dividing the superior oblique was to cause the 
eye to roll more forcibly upwards ; and if we suppose that 
the influence of the fourth nerve is, on certain occasions, to 
cause a relaxation of the muscle to which it goes, the eye- 
ball must be then rolled upwards.* 
* The nerves have been considered so generally as instruments for stimulating 
the muscles, without thought of their acting in the opposite capacity, that some 
additional illustration may be necessary here. Through the nerves is established 
the connection between the muscles, not only that connection by which muscles 
