on the Structure, Distribution, and Functions of the Nerves. 445 



motor oculi, given by Willis, although his reasons for calling 

 it so were fanciful and unsatisfactory. The fifth nerve, by its 

 ophthalmic division, gives branches to the muscles of the eye, 

 but not so profusely as to the surrounding parts ; and not more 

 than sufficient to give them sensibility in the degree possessed 

 by muscular substance generally. Since the branches of this 

 fifth nerve, transmitted to the muscles of the eyelids and fore- 

 head, do not minister in any degree to muscular action there, 

 it would be unwarrantable to suppose that they served the 

 purpose of giving action to the muscles within the orbit. For 

 these reasons, I conceive the third nerve to be that which 

 gives volition to the muscles of the eye, and that it is, of all 

 the nerves of the body, the most perfectly and directly under 

 the power of the will. 



"The sixth nerve is called abducens, and motor externus. With 

 regard to its origin and distribution, there is no obscurity in 

 this nerve ; it arises from the same track of medullary matter 

 which gives rise to the motor nerves, and it is distributed to 

 a voluntary muscle, the rectus externus. In this respect it is 

 like a subdivision of the third, and without doubt it is a vo- 

 luntary nerve ; but there is a circumstance in its connexion 

 which I cannot explain. It receives a gross branch from the 

 great visceral nerve called Sympathetic. This nerve, ascend- 

 ing 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 connexion, 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 ne- 

 vertheless 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 pro- 

 per office. In our books of Anatomy, the nerves are num- 

 bered 



