It is evident from an examination of the origin, course, and insertion of the rec., i muscles, that each of them acting singly is capable of making the eye-ball revolve in its own direc tion ; the superior rectus directs the cornea upwards; the inferior rectus antagonises the superior and directs the cornea downwards; the external rectus directs it outwards and is 7 antagonised by the internal, which draws the , cornea inwards towards the nose. It is also evident that the action of any two contiguous recti muscles will give the comea a direction intermediate between that which it would as sume from the action of each of them singly; the superior and internal recti acting together, will direct •the cornea upwards and inwards, while the inferior and external will direct it ' downwards and outwards ; so that the cornat may be made to assume any intermediate posi-, tion by the action of the recti muscles alone. , The successive action of all the recti mus-, cles would produce a movement of the eye-, ball analogous to cireumduction of a limb; and as the circumduction of a limb is a move;ii ment altogether distinct from rotation, so i this circumduction of the eye-ball entirely dis-,4 tinct from any rotation upon its antero-postcrio4 axis. In circumduction the centre of the! cornea describes a circle, whereas in rotation, this point remains fixed, forming the anteriot, extremity of an imaginary axis, round which the circumference of the cornea revolves. is of much imporLance to have a definite notion of each of these movements, as we shal! thus avoid one source of confusiou in con5 sidering the action of the straight and oblique muscles of the eye. 'laving thus defined rotation, no argument is necessary to prove that the mai. muscles are incapable of produclas such a movement; a glance at their directioa 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 heing 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 retmct, 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 rnuscles; 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 i of antagonism which the muscles on the op posite sides of the face exert upomeach other. Paralysis of the_portio dura on one side is . ttended with a traction of the features to the qaposite side ; this results from the ordinary onicity or passive contraction of the muscles n the one stde, unopposed by the correspond ng force on the other ; the distortion is gene 'ally conspicuous enough when the muscles are It t rest, but when they are thrown into active ontraction it becomes still more marked, nd the movements of the sound side are un Iteady and oscillating. During the healthy tate then the symmetry of the features is 1 fliia iofipt apionsei dt e bs ircl e"s1 i so fa tnhtea gfoa enei s rn ionf itihkee nmivastIcnI ee sr Olen the muscles are at rest, the eye-ball is 'ept delicately balanced between its six es; the superior rectus opposes the inferior, _A id the external opposes the internal, NOtile 1 the obliqui are opi)osed to each other, and the recti conjointly are_autagonised in their retracting tendency by the opposite force of the obliqui. This is the condition during a state
of rest, when the contraction of' all the mus cles is merely that of their ordinary tonicity or passive contraction. Now, suppose one straight 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 eye7ball 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 otttwards ; on the other hand, according to Muller, Mcinro, 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 disagyeement as to the direction in which circnmduction occurs under the influence of these muscles, is of itself an arg-ument 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 eyelof a markkey. 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, on certain oemsions, to cause a relaxation of the muscle to which it goes, the eye-ball must be then rolled upwards."• Sir C. Bell adduces no proof that the involun tary movements which he mentions are performed by the oblique muscles; on the contrary they may all be effected by the straight muscles. The fact that these movements are involuntary is no argument against their being produced by muscles, which under ordinary circumstances are strictly voluntary. Thus, Sir C. Bell says, when the eye is exposed and irritated, the cornea is directed upwards to a greater extent than can be done by a voluntary effort. This probably is the case, but we need not have recourse to the oblique muscles in order to explain it. Under the influence of the irri tation applied to the eye the superior rectus contracts violently in order to elevate the cornea beneath the upper lid, aud thus to remove it from danger ; precisely in the same manner under the irritating influence of strumous ophthalmia the orbicularis muscle contracts with a spasmodic force much exceeding that of any voluntary contraction of that muscle. Both oblique muscles have the striated struc ture of voluntary muscles, and the inferior oblique receives a branch from the third nerve, all the other muscles supplied by which are known to be voluntary in their action. NVe may, therefore, dismiss the idea that the ob lique muscles are specially concerned in pro ducing the involuntary movements of the eye.