on the Structure, Distribution, and Functions of the Nerves. 44.1 



per on this subject, took occasion to observe that the shutting 

 of the eyelids was not the only part of this act of preservation, 

 for the motions of these are at the same time attended with a 

 rolling of the eye-ball. The question, how is this relation be- 

 tween the eyelids and eyeballs established ? leads therefore to 

 an examination of the fourth nerve. 



" The fourth Nerve. 



" This is a fine nerve, which take 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 without 

 touching them, and is entirely given to one muscle, the su- 

 perior 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 its root, or source, 

 different from the other nerves, from the nerve of vision, the 

 nerve of common sensibility, and the nerve of voluntary mo- 

 tion ? 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 relaxa- 

 tion of the muscle to which it goes, the eyeball must be then 

 rolled upwards*. 



" The course of inquiry leads us, in the next place, to ob- 

 serve the vicinity of the root of this fourth nerve, to the origin 

 of the respiratory of the face, and we find them arising from 

 the same track of fibrous substance. The column of medul- 

 lary matter which constitutes that part of the medulla oblon- 

 gata from which the respiratory nerves arise, terminates up- 



* * The nerves have been considered so generally as instruments for stimu- 

 lating 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 connexion between the muscles, not only that connexion 

 by which muscles combine to one effort, but also that relation between the 

 classes of muscles by which the one relaxes while the other contracts. I 

 appended a weight to a tendon of an extensor muscle, which gently stretch- 

 ed it ami drew out the muscle j and I found that the contraction of the op- 

 ponent flexor was attended with a descent of the weight, which indicated 

 the relaxation of the extensor. To establish this connexion between two 

 elates of muscles, whether they be grouped near together, as in the limbs, 

 or scattered widely as the muscles of respiration, there must be particu- 

 lar and appropriate nerves to form this double bond, to cause them to con- 

 spire in relaxation as well as to combine in contraction. If such a rela- 

 tionship be established, through the distribution of nerves, between the 

 muscles of the eyelids and the superior oblique muscle of the eyeball, the 

 one will relax while the other contracts." 



3 K 2 wards, 



