Further, it is our firm conviction that the oblique muscles are in no way concerned in cireumduction of the eye-ball; that they nei ther abduct nor adduct, neither raise nor depress the comea, nor do they produce any of the in termediate movements. The following are the circumstances which appear to us to favour this conviction. 1st. Both oblique, muscles pass outwards almost at right angles with the recti muscles, and are inserted close upon a line intermediate between the anterior and pos terior half of the eye-ball : this direction and insertion are evidently most unfavourable for the production of any of the above-mentioned movements. 2d. Those who assert that the oblique muscles have the power of circum ducting the eye make the most contradictory statements as to the direction which the eye assumes under their influe.nce. These opposite statements are snfficiently accounted for when we consider that they are founded on the re sults of traction on the oblique muscles after death, when the fat and other parts in the orbit liave become firm and unyielding, and the steadying influence arising from the antagonism of the other muscles has ceased. 3d. The recti muscles are of themselves capable of cir cumducting the comea in all directions ; this is evident from their direction and insertion, and was proved by Sir C. 13ell's experiments above-mentioned. 4th. There is an important movement of the eye-ball which can be effected by no other than the oblique muscles. and for the production of which in all probability these muscles are provided : the movement to which we refer is rotation of the eye upon its antero posterior axis. ' The true use of these muscles we believe to have been pointed out by John Hunter in a paper on the use of the oblique,muscles, in his " Observations on certain Parts rif the Animal Economy." Ile first explains that for perfect vision it is essential that when we are examining an object, any motion of the object or of our own bodies should be so counter acted by the movements of the eye-ball that the image of the object may be kept on the same point of the retina, and not be allowed to move over its surface. We have a familiar illustration of this when we keep our eyes motionless and fixed on the ground, while moving rapidly in a airriage ; the surface of the road appears confused and the stones ar ranged in lines, as their images pass rapidly over the retina: it is only when we allow the eye to follow these objects that we have a dis tinct perception of any of them. Bunter then g,oes on to explain the use of the oblique muscles. " To prevent any prooTessive motion of the object over the retina ofc.the eye, either from the motion of the object itself, or of the head in some motions of that part, the straight muscles are provided, as has been explained ; but the effects, which wOuld arise from some other inotion of the head, as from shoulder to shoulder,* cannot be corrected by the action of the straight muscles, therefore the oblique muscles are provided. Thus, when we look at an object and at the same time move o head to either shoulder, it is moving in arch of a circle whose centre is the neck, a of course, the eyes would have the same qu. tity of motion on this axis if the oblique m cles did riot fix them upon the object. Wli the head is moved towards the right shoul the superior oblique muscle of the right si acts, and keeps the right eye fixed OD t object, and a similar effect is produced ui the left eye by the action of its inferior obliq muscle. When the head moves in a cont direction, the other oblique muscles prod the same effect." If we again consider the direction and tion of the oblique muscles, it is evid they are intended for the office which 1 has assigned them. Passing outward above, the other below, in a direction al right angles with the antero-posterior the eye-ball to their insertion near its their action must obviously be to ro eye upon that axis. The reason of thei quity probably is that their direction backwil as well as outwards, by enabling them to gonise the straight muscles, more certainly cures the delicate suspension or the eye-ball when the muscles are at rest, and a steady movement of it when any of them are thrown into action. We have the following experi mental evidence to offer in support of the state ment which Bunter has made. A dog was killed by the injection of air into a vein, and immediately the inferior oblique muscle was exposed by dissecting off the conjunctiva, with out in any way interfering with the surrounding parts ; by means of two fine wires a slight electric current was then directed through the muscle. The effect was a rapid rotation of the eye upon its antero-posterior axis, so that a piece of paper placed at the outer margin of the cornea passed downwards and then inwards towards the nose. The superior oblique was then exposed at the back of the orbit, and was treated in the same manner. The rotatory movement produced was precisely the reverse of the former; the paper at the outer margin of the cornea passed upwards, and then in wards towards the nose. In tbe case of the superior oblique the movement was less exten sive, the irritability of the muscle being less, perhaps from the delay in exposing it, and from some slight injury inflicted on it in so doing. There could be no doubt as to the direction of the movement in both cases; there was not the slightest appearance of elevation, depression, abduction, or adduction of the cornea. This expeliment was witnessed by Dr. Todd and Mr. Bowman. The experiment was subse quently repeated on another dog with precisely the same result. The superior oblique in the second experiment did not contract so vigo rously as the inferior, but the movement it pro duced was the same as in the first experiment; and when gentle traction was made in the pos terior part of the muscle, the rotation of the eye was very decided, and in a direction the reverse of that in which it rotated under the influence of the inferior oblique; ag,ain there was not the slightest movement of circumduction. There
can be no doubt that the function of these muscles is the same in all 'animals in which they exist ; and any experiments to determine their use must be more satisfactory when per formed on animals immediately after death than in the human subject at a considerable period after death, when the fat and muscles have become equally firm and unyielding. Under such circumstances it is evident that the results of traction upon the muscles cannot be relied upon as accurate. It is remarkable that this rotation of the eye should have excited so little attention; since, if we only recognise the existence of such a movement, the use and necessity of the oblique muscles must be ac knowledged, it being evident, as we have pre viously stated, that the straight muscles are incapable of effecting it.
Consensual movements of the two eyes.— Upon this subject we subjoin the following extract from There is an innate tendency and irresistible impulse in the cor responding branches of the third nerve to asso ciate action; whilein the sixth nerve, not only is this tendency absent,but the strong action of one of these nerves is incoinpatible with the action of the other. These innate ten dencies in the third and sixth nerves are ex tremely important for the function of vision, for if, in place of the sixth nerves, the external recti muscles had received each a branch of the third nerve, it would have been impossible to make one of these muscles act without the other; one eye, for example, could not have been directed inwards while the other was di rected outwards, so as to preserve the paral lelism or convergence of their axes, but they would necessarily have diverged when one rectus externus had been made to act volunta rily. To render possible the motion of one eye inwards While the other is directed out wards, the external straight muscles have re ceived nerves which have no tendency to con sensual action. In consequence, however, of the tendency in the two internal straight mus cles to associate action, it is necessary, where one eye is directed inwards and the other out wards, that the contraction of the rectus cx ternus of the latter should be so strong as to overcome the associate action of the rectus inter nus of the same eye; and in the effort to direct one eye completely outwards we actually feel this stronger contraction of the external rectus." It is certainly contrary to our general notions of the skill and economy of nature to sup pose that she would so clumsily construct and endow the muscles and nerves of the eye, that in order to direct one eye outwards the external rectus muscle must struggle with and overcome the internal rectus of the same eye in conse quence of this " irresistible impulse in the cor responding branches of the third nerve to associate action." Doubtless the generality of those who have no theory to support will acknow ledge that in directing the eye outwards they are unconscious of any such struggle between opposing muscles as is here supposed, and that abductionlof the etrt is attended with as little effort as either its elevation or depression. There is then no such irresistible tendency to associate action between the branches of the third nerve supplied to the internal recti muscles. Both internal recti muscles may be made to act at the same time, and thus to produce a con vergence of the optic axes ; and this being an unnatural position of the eyes is attended with a considerable and a painful effort, each inter nal rectus having to overcome the external rectus of the same eye, which has a tendency to consentaneous action with the external rectus of the other eye. That the external rectus must have the advantage in any struggle between it and the internal rectus is evident from the greater thickness and consequent strength of the former muscle. The only muscles supplied by the third nerve in which this tendency to con sentanecus action is irresistible, are the superior and inferior recti of both eyes; we cannot pos sibly raise one eye without at the same time raising the other, nor can we depress one eye without a corresponding movement of the other. Then, as we hate seen, there is no tendency to consentaneous action between the branches of the third nerve supplied to the internal recti muscles, and it is only by a considerable effort that they can be made to act togetlter ; whilst the branches of the satne nerve supplied to the levatores palpebrarum, which for the most part act consentaneously, may by a very slight effort of the will be made to act separately ; but few persons experience any difficulty in opening one eye while the other is closed. Again, the tendency to consentaneous action betu eel, tlie internal rectus supplied by the third nerve and the external rectus supplied by the sixth nerve, as well as between the inferior oblique supplied by the third nerve and the superior oblique by the fourth, is as irresistible as that between the supe rior and inferior recti of the two eyes. We see then that all the muscles supplied by corres ponding bmnches of the third nerve have not this tendency to consensual action, and two muscles supplied by the third nerve act con sentaneously with two other muscles supplied by the fourth and the sixth nerves respectively.