MUSCLES OF THE ORBIT. 277 



Nerves. — Five of the muscles of the orbit, viz., levator palpebrje supe- 

 rioris, the superior, internal, and inferior recti and the inferior oblique, are 

 under the influence of the third pair or common oculo-motor nerve ; the external 

 rectus is supplied by the sixth pair, or abducent ocular, and the superior oblique 

 by the fourth pair, or trochlear nerve. From what is stated hereafter as to the 

 action of the muscles, it vnll api)ear that in all the consentaneous movements of 

 du-ection of the two eyes muscles are in action which receive their motor nerves 

 from two different sources. 



Actions. —The levator palpebrfe superioris is, as its name implies, simply an 

 elevator of the upper eyelid, acting in this as the antagonist of the upper pal- 

 pebral part of the orbicularis muscle. 



The eyeball Ls so situated in the structures which surround it in the orbit that 

 it is capable of free motion on a central fixed point : but it does not appear to 

 shift its place as a whole within the orbit, nor to undergo perceptible change of 

 fonn from the action of the muscles. The position of the point round which 

 the movements of the eyeball take place is nearly in the centre of cur\'ature of the 

 posterior wall, and from half a line to a line behind the middle of the antero- 

 posterior axis of the eyeball. 



The movements of the eyeball may be conveniently reduced to foiu- kinds, viz., 

 1, simple lateral movements in a hoiizontal plane ; 2, simple movements of eleva- 

 tion or depression ; .3, oblique movements of elevation or depression, and 4, 

 movements of rotation. In the first two kinds the vertical meridian of the eye 

 is not subject to any change of inclination ; in the third kind the movements of 

 direction are accompanied by a small amount of rotation or inclination of the 

 vertical meridian to one or other side : and in the fourth kind, when simple, the 

 vphole movement is one of inclination of the vertical meridian. These movements 

 however, unless perhaps the first, are seldom simple, but more frequently different 

 kinds are combined together. The three first kinds constitute the various move- 

 ments of direction by which the \'isual axis is turned mthin certain limits to 

 various points in space ; the extent of motion being about 'JO^ in the vertical and 

 100° in the horizontal direction. Simple movements of rotation do not appear to 

 occur to any considerable extent, and it has been ascertained by experiment that 

 they do not, as was supposed by Hueck and others, maintain the eyeballs in a 

 fixed position during inclined movements of the head 



In these different movements the six muscles of the eyeball may aavan- 

 tageously be considered as acting in three pairs. 1st. In the horizontal move- 

 ments the internal and external recti muscles are the sole agents, the one acting 

 as an adductor o.nd the other as an abductor ; and this movement they effect 

 without any rotation, their line of action being exactly in the horizontal plane 

 of the eyeball. 2nd. It is different with the superior and inferior recti ; for while 

 these muscles undoubtedly are respectively the most dii-ect elevators and de- 

 pressors of the cornea, they have both a tendency, from the line of their action 

 being to the inside of the centre of motion of the eyeball, to produce inward 

 direction with a small amount of rotation. This tendency is corrected liy the 

 association of the oblique muscles in all upward and downward movements : the 

 inferior oblique being associated -^-ith the superior, and the superior oblique 

 with the inferior rectus muscle. The simple action of the superior ol^lique 

 muscle, when the eye is directed straight forAvard, is to produce a movement of 

 the cornea downwards and outwards, that of the inferior oblique to dii'ect the 

 coiiiea upwards and oiit^'ards, and in both with a cei"tain amount of rotation, 

 though in different directions in the two cases. But these movements caused 

 by the oblique muscles are precisely those which are required to neutralise the 

 inward direction and rotatory movements produced by the superior or the 

 inferior rectus, and accordingly, by the combined action of the superior rectus 

 and the inferior oblique muscles a straight upward movement is effected, while 

 a similar effect in the dowiiward direction results from the combined action of 

 the inferior rectus and superior oblique muscles. 



It has been further sho'ma that in all the oblique movements of direction a 

 combination takes place of the action of the obliqtie with that of the recti 

 muscles. Here, however, two recti muscles are in action and are associated with 

 one oblique muscle ; as, for example, in tho upward and inward direction, the 



