ORBIT. 



789 



and their position with regard to the eye-ball 

 will at once determine this point. If then any 

 such movement occurs, other muscles must be 

 provided in order to effect it. 



We now proceed to consider the action of 

 the oblique muscles. 



It may be well to remind the reader that, in 

 all the vertebrate animals, these muscles have 

 essentially the same direction and relations; 

 the only difference being that in fishes, rep- 

 tiles, and birds, the superior oblique arises 

 from the anterior part of the orbit, whence it 

 passes backwards and outwards to its insertion ; 

 whereas, in Mammalia, it comes from the pos- 

 terior part of the orbit and passes through a 

 tendinous pulley before taking its course back- 

 wards and outwards ; the action of the muscle 

 will obviously be the same in both cases. One 

 function which has been assigned to the oblique 

 muscles, is that of antagonising the recti so as 

 to prevent the retraction of the eye-ball within 

 the orbit during the action of the latter mus- 

 cles. To this conclusion Sir C. Bell asserts 

 there are many objections : two of these ob- 

 jections we subjoin in his own words. "1. 

 In creatures where the eye is socketed in a 

 cup of cartilage and cannot retract, the oblique 

 muscles are nevertheless present. 2. Where a 

 powerful retractor muscle is bestowed in ad- 

 dition to the recti muscles to pull the eye-ball 

 back, the oblique muscles have no additional 

 magnitude given to them to pull the eye-ball 

 forwards." Now we must not suppose that the 

 antagonism exerted by the oblique muscles is 

 such as to oppose the conjoint forcible action, 

 or active contraction, of all the recti muscles, 

 and of a retractor when such a muscle exists. 

 If such were the case, we should certainly find 

 the developement of the oblique muscles in 

 some degree proportioned to that of the mus- 

 cles they were intended to antagonise, and a 

 cup of cartilage at the back of the eye-ball 

 would apparently supersede the necessity for 

 any antagonism on the part of the oblique 

 muscles ; but the kind of antagonism which 

 the oblique muscles probably exert upon the 

 recti is equally necessary whether the eye-ball 

 be encased in cartilage or supplied with a re- 

 tractor muscle. It is simply the same kind 

 of antagonism which the muscles on the op- 

 posite sides of the face exert upon each other. 

 Paralysis of the portio dura on one side is 

 attended with a traction of the features to the 

 opposite side ; this results from the ordinary 

 tonicity or passive contraction of the muscles 

 on the one side, unopposed by the correspond- 

 ing force on the other ; the distortion is gene- 

 rally conspicuous enough when the muscles are 

 at rest, but when they are thrown into active 

 contraction it becomes still more marked, 

 and the movements of the sound side are un- 

 steady and oscillating. During the healthy 

 state then the symmetry of the features is 

 maintained by this antagonism of the muscles 

 on opposite sides of the face. In like manner 

 when the muscles are at rest, the eye-ball is 

 kept delicately balanced between its six mus- 

 cles; the superior rectus opposes the inferior, 

 and the external opposes the internal, while 



the obliqui are opposed to each other, and 

 the recti conjointly are antagonised in their 

 retracting tendency by the opposite force of the 

 obliqui. This is the condition during a state 

 of rest, when the conttaction of all the mus- 

 cles is merely that of their ordinary tonicity or 

 passive contraction. Now, suppose one stiaight 

 muscle to be thrown into a state of voluntary 

 active contraction; immediately the cornea is 

 directed towards that muscle, the antagonism 

 of the other five muscles serving the important 

 purpose of preventing any irregular oscillatory 

 movement of the eye-ball ; when the contrac- 

 tion of that muscle ceases, the eye is at once 

 restored to its original position. One of the 

 uses of the oblique muscles then is by their 

 antagonism of the recti to assist in preventing 

 any unsteady motion of the eye-ball. This, 

 however, is by no means the only or the chief 

 use of the oblique muscles, and the question 

 arises, what movements of the eye-ball are 

 effected by the contraction of these muscles? 

 Upon this subject the most contradictory state- 

 ments have been made; on the one hand 

 Scemmering, Cloquet, and Harrison assert, that 

 the superior oblique directs the pupil down- 

 wards and inwards, the inferior oblique moves 

 it upwards and outwards ; on the other hand, 

 according to Miiller, Monro, and Sir C. Bell, 

 the superior oblique directs the pupil down- 

 wards and outwards, the inferior oblique up- 

 wards and inwards. All these anatomists 

 agree in supposing that the oblique muscles 

 effect what we have called circumduction of 

 the eye-ball, but their disagreement as to the 

 direction in which circumduction occurs under 

 the influence of these muscles, is of itself an 

 argument against the probability of any such 

 movement being produced by them. We have 

 before stated that the recti muscles are of them- 

 selves capable of circumducting the eye in all 

 directions : this was admitted and proved ex- 

 perimentally by Sir C. Bell. He " cut across 

 the tendon of the superior oblique muscle of 

 the right eye of a monkey. He was very little 

 disturbed by this experiment, and turned 

 round his eyes with his characteristic inquiring 

 looks, as if nothing had happened to affect the 

 eye." In another experiment he " divided the 

 lower oblique muscle of the eye of a monkey. 

 The eye was not, in any sensible manner, af- 

 fected ; the voluntary motions were perfect 

 after the operation." The result of these ex- 

 periments appeared to Sir C. Bell to confirm 

 the opinion which he entertained that the 

 oblique muscles perform certain involuntary 

 movements, such as the forcible elevation of 

 the cornea under the upper lid when the eye 

 is irritated, and the rolling of the cornea under 

 the lid when the eye is closed. He appears 

 anxious to prove that the fourth nerve presides 

 over the upward movement of the eye-ball 

 which he says occurs during sneezing and 

 certain other respiratory movements ; but as he 

 has previously stated that the superior oblique 

 to which the fourth nerve is distributed turns 

 the eye downwards and outwards, in order to 

 reconcile the two views he says, " if we sup- 

 pose that the influence of the fourth nerve is, 



