ORBIT. 



791 



cures the delicate suspension of 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 Hunter 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 the 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 

 experiment 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; again 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 Muller:* "There is an innate 

 tendency and irresistible impulse in the cor- 

 responding branches of the third nerve to asso- 



* Physiology, by Dr. Baly, p. 99. 



ciate action ; while in the sixth nerve, not only 

 is this tendency absent, but the strong action 

 of one of these nerves is incompatible 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 ex- 

 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 

 abduction of the eye 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- 

 sentaneous 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 have seen, there is no tendency to 



