MOVEMENTS OF THE EYEBALL. 835 



two Oblique muscles. The precise actions of these are not easily established 

 by experiment on the lower animals; for in all those which ordinarily main- 

 tain the horizontal position, there is an additional muscle, termed the re- 

 tractor, which embraces the whole posterior portion of the globe, and passes 

 backwards to be attached to the bottom of the orbit. 1 If the origin and 

 insertion of the four Recli muscles be examined, however, no doubt can re- 

 main that each of them, acting singly, is capable of causing the globe to 

 revolve in its own direction, the superior 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 inter- 

 mediate position may thus be given to the eyeball by these muscles alone. 

 In two directions, however, their combined action is supplemented by the 

 oblique muscles. The superior oblique rotates the eye downwards and out- 

 wards ; the inferior oblique upwards and outwards. Another important 

 operation of the oblique muscles is to maintain the parallelism of the verti- 

 cal meridians of the two eyes in certain movements, for it is found that 

 whilst the external and internal recti merely turn the eyes outwards and 

 inwards, without deflecting the vertical meridian, the superior and inferior 

 recti, in consequence of the obliquity of their muscle planes, not only rotate 

 the eye respectively upwards and downwards, but deflect the upper extrem- 

 ity of the vertical meridian, the former inwards, the latter outwards. But 

 the oblique muscles also deflect the vertical meridian, the superior turning 

 the upper extremity of the vertical meridian inwards, the inferior out- 

 wards, and therefore by co-operating with the recti, are able to preserve the 

 parallelism of the vertical meridians of the two eyes, as shown in the follow- 

 ing table: 



Movement Is effected by the 



Upwards, Superior rectus, aided by the inferior oblique. 



Downwards, Inferior rectus, aided by the superior oblique. 



Inwards, Internal rectus. 



Outwards, External rectus. 



TT f Superior and internal recti muscles, aided by the inferior 



Upwards and inwards, .{ ',,. ' 



I oblique. 



TT ( Superior and external recti muscles, aided by the inferior 



Upwards and outwards, . j oblique. 



, f Inferior and internal recti muscle?, aided by the superior 

 Downwards and inwards, -; ,,. 



Downwards and outwards, Inferior and external recti, aided by the superior oblique. 



684. On studying the Voluntary movements of the Eyeballs, we are led 

 to perceive that they are not so much symmetrical as harmonious that is to 

 say, the corresponding muscles on the two sides are rarely in action at once; 

 whilst such a harmony or consent exists between the actions of the muscles 

 of the two orbits, that they work to one common purpose, namely, the direc- 

 tion of both eyes toAvards the required object. They may be arranged under 

 two groups ; the first comprising those which are alike harmonious and sym- 

 metrical ; the second including those which are harmonious but not symmet- 

 rical. To the first group belong the following: 1. Both eyeballs are elevated 

 by the contraction of the two Superior Recti. 2. Both eyeballs are depressed 

 by the conjoint action of the Inferior Recti muscles. 3. Both are drawn 

 directly inwards, or imvards and downwards, as when we look at an object 

 placed on or near the nose : this movement is effected by the action of the 

 Internal Recti of the two sides, with or without the Inferior Recti. It is 



1 See Alfred v. Grafe, Klinische Analyse der Motilitats-storungen des Auges, and 

 v. Grafe's Archiv f. Ophthalmologie, Bd. i-iii. 



