2 99 
nerves of the orbit of the eye. 
a voluntary muscle, the rectus externus. In this respect it is 
like a subdivision of the Third, and without doubt it is a 
voluntary nerve ; but there is a circumstance in its connec- 
tion which I cannot explain. It receives a gross branch from 
the great visceral nerve called Sympathetic. This nerve, 
ascending through the base of the skull, unites with the Sixth 
nerve as it is entering the orbit. Some having proceeded so 
far, would be inclined to call this an accidental connection, 
and so leave it ; but similar investigations for many years 
have brought me to the conviction that there is no accident 
in an animal body ; and Comparative Anatomy proves this 
to be a regular established relation. 
To return to the consideration of these nerves of volition 
as they regard the eye, we may affirm, that although they 
want sensibility in the common acceptation of the term, they 
no doubt furnish the mind with the rudiments of certain sen- 
sations, and so far resemble the nerves of the senses. From 
experiments narrated in the first part of this paper, it appears, 
that we are sensible to the degree of agency exercised by the 
voluntary muscles of the eye. These nerves, the Third and 
Sixth, although they receive no external impression, are 
nevertheless agents which give rise to the perceptions of 
place or relation, in aid of that sensibility enjoyed by the optic 
nerve and retina. 
I hope I have now unravelled the intricacy of the nerves 
of the head, and have correctly assigned to each nerve its 
proper office. In our books of Anatomy, the nerves are 
numbered according to the method of Willis, an arrange- 
