SYMMETRY AND HARMONY OF MUSCULAR MOVEMENTS. 913 



ticularly on the Movements of the Eye, as presenting certain points of peculiar 

 interest, some of which have an important bearing on Surgical practice. It will 

 be recollected that, in the Human Orbit, six muscles for the movements of the 

 eyeball are found the four Recti, and the two Oblique muscles. The precise 

 actions of these are not easily established by experiment on the lower animals; 

 for in all those which ordinarily maintain the horizontal position, there is an 

 additional muscle, termed the retractor, which embraces the whole posterior por- 

 tion of the globe, and passes backwards to be attached to the bottom of the 

 orbit. 1 If the origin and insertion of the four Recti muscles be examined, how- 

 ever, 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 further 

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

 must 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 at- 

 tention it deserves, leads us to perceive that the Oblique muscles 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 perform- 

 ance of diagonal movements by the Recti muscles, which renders 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 j and that, as we have good reason to believe these movements to 

 be effected by the combination of the Recti muscles, there is no reason why the 

 other diagonal movements should not also be due to them. 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 con- 

 firmation from the experiments of Dr. G. Johnson. 2 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 this is put in force, 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 tfee objects upon which our gaze is fixed 

 still to fall upon the same parts of the retinae, 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 pre- 

 serve the fixed position of the eyeball, and consequently to keep the image of 



creased if we half shut both eyes, and then try to close one and to open the other. So if 

 we try to move our two hands, as if they were simultaneously winding cord in opposite direc- 

 tions upon two reels placed in front of us, we shall find ourselves unable to do so without 

 a constant exercise of the attention, and even then but slowly and with difficulty ; although 

 the very same movements may be separately performed, or a movement of both hands in 

 the same direction, with the greatest facility. 



1 This muscle is most developed in Ruminating animals, which, during their whole time 

 of feeding, carry their heads in a dependent position. In most Carnivorous animals, instead 

 of the complete hollow muscular cone (the base inclosing the eyeball, whilst the apex sur- 

 rounds 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 operations which we may perform upon the 

 other muscles of the Orbit ; and, 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 animals of the order Quadrumana, most nearly 

 allied to Man, would be more satisfactory ; as in them, the retractor muscle is almost or 

 entirely absent. 



2 "Cyclopaedia of Anatomy and Physiology," vol. iii. p. 790. 



58 



