CONSENSUAL MOVEMENTS OF THE EYE. 345 



vorous animals, instead of the complete hollow muscular cone (the base inclos- 

 ing the eyeball, whilst the apex surrounds the optic nerve) which we find in 

 the Ruminants, there are four distinct strips, almost resembling a second set 

 of recti muscles, but deep-seated, and inserted into the posterior instead of the 

 anterior portion of the globe. It is obvious that the actions of these must 

 greatly affect the results of any operation, which we may perform upon the 

 other muscles of the Orbit; arid, as it is impossible to divide the former, 

 without completely separating the eye from its attachments, we have no means 

 of correcting such results, but by reasoning alone. Experiments upon ani- 

 mals of the order of Quadrumana, most nearly allied to Man, would be more 

 satisfactory ; as in them, the retractor muscle is almost or entirely absent. 

 If the origin and insertion of the four Recti muscles be examined, however, 

 no doubt can remain that each of them, acting singly, is capable of causing 

 the globe to revolve in its own direction, the superior rectus causing the pupil 

 to turn upwards, the internal rectus causing it to roll towards the nose, and 

 so on. A very easy and direct application of the laws of mechanics will fur- 

 ther make it evident to us, that the combined action of any two of the Recti 

 muscles will cause the pupil to turn in a direction intermediate between the 

 lines of their single action; and that any intermediate position may thus be 

 given to the eyeball by these muscles alone. This fact, which has not 

 received the attention it deserves, leads us to perceive, that the Oblique mus- 

 cles must have some supplementary function. It may be objected that this is 

 a theoretical statement only ; and that there may be some practical obstacle 

 to the performance of diagonal movements by the Recti muscles, which ren- 

 ders the assistance of the Obliques essential for this purpose. But to this it 

 may be replied, that no single muscle can direct the ball either downwards 

 and inwards, or upwards and outwards; and that, as we have good reason to 

 believe these movements to be effected by the combination of the Recti mus- 

 cles, there is no reason why the other diagonal movements should not also be 

 due to them. 



451. The most probable account of the functions of the Oblique muscles of 

 the eye, seems to be that which was long ago suggested by John Hunter, and 

 which has received confirmation from the recent experiments of Dr. G. John- 

 son.* It has been just shown that the action of the Recti muscles upon the 

 pupil, is such as to cause it to revolve in any given direction; and they are 

 put in action, not merely to alter the range of vision, the head remaining 

 stationary, but also to keep the range of vision the same, and to cause the 

 images of the objects, upon which our gaze is fixed, still to fall upon the 

 same parts of the retinre, by maintaining the position of the eyes when the 

 head is moved upwards, downwards, from side to side, or in any intermediate 

 direction. But these muscles are not able to rotate the eyeball upon its antero- 

 posterior axis; and such rotation is manifestly necessary to preserve the fixed 

 position, of the eyeball, and consequently to keep the image of the object un- 

 der survey upon the same part of the retina, when the head is inclined side- 

 ways, or bowed towards one shoulder and then towards the other. It appears 

 from the experiments of Dr. G. Johnson, that the action of the Oblique mus- 

 cles is exactly adapted to produce such a rotation; the Inferior oblique, in its 

 contraction, causing the eyeball to move upon its antero-posterior axis, in such 

 a manner that a piece of paper, placed at the outer margin of the cornea, passed 

 downwards and then inwards towards the nose; and the Superior oblique 

 effecting precisely the reverse action, the paper at the outer margin of the cor- 

 nea passing first upwards and then inwards. There was not the slightest 

 appearance, in these experiments, of elevation, depression, abduction, or ad- 



* Cyclopedia of Anatomy and Physiology, vol. iii. p. 790. 



