THE MUSCLES OF THE ORBIT. 



291 



Varieties. The levator palpebrae sometimes gives off a distinct muscular slip from its 

 inner border to the trochlea, forming the tensor troctilece of Budge. Absence of the levator 

 palpebras has been observed. 



The muscles of the eye seldom vary. The external rectus has been seen : with its two 

 heads separate to their insertion, thus forming a double muscle ; and giving slips to the 

 outer wall of the orbit and the lower eyelid. Eex has described under the name of 

 obliquus inferior accessorius a muscular slip passing from the inferior rectus to the inferior 

 oblique. 



The transversus orbits (Bochdalek) is an arched slip of muscular fibres passing from the 

 orbital plate of the ethmoid across the upper surface of the eyeball to the outer wall of the 

 orbit. 



Nerves. Five of the muscles of the orbit, viz., levator palpebrse superioris,~the superior, 

 internal and inferior recti, and the inferior oblique, are under the influence of the third or 



Fig. 273. DIAGRAM SHOWING THE AC- 

 TIONS OF THE OCULAR MUSCLES. 

 (Landois. ) 



S, S 1, sagittal axis of the eyeball 

 (visual axis) ; Q, Q 1, transverse axis of 

 the eyeball ; the directions of traction of 

 the several muscles are indicated by the 

 thick lines ; the dotted lines R. inf. , R. 

 sup., and Obi. sup., ObL inf., represent 

 the axes about which the superior and in- 

 ferior recti and obliqui muscles i-espec- 

 tively move the eyeball ; the axis of 

 rotation of the internal and external 

 recti is perpendicular to the plane of 

 the paper, and identical with the vertical 

 axis of the eyeball, 0. 



common oculomotor nerve ; the ex- 

 ternal rectus is supplied by the sixth 

 or abducent ocular, and the superior 

 oblique by the fourth or trochlear 

 nerve. 



Actions. The levator palpebrce 

 superioris is simply an elevator of 

 the upper eyelid, acting as the an- 

 tagonist of the upper palpebral part 

 of the orbicularis muscle. 



The eyeball is 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, at least to any 

 extent, nor to undergo perceptible change of form 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 curvature 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 four kinds, viz., 1, simple 

 lateral movements in a horizontal plane ; 2, simple movements of elevation or depression ; 3, 

 oblique movements of elevation or depression ; and 4, movements of rotation about a sagittal 

 axis. 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 inclination of the vertical meridian to one or other side ; and in the fourth kind, when 

 simple, the whole 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 first three kinds constitute the various movements of direction by 

 which the visual axis is turned within certain limits to various points in space, the extent of 

 motion from the primary position (with the visual axis horizontal and parallel to the median 

 plane) being about 42 outwards, 45 inwards, 34 upwards, and 57 downwards. Simple 

 movements of rotation do not appear to occur to any considerable extent, and it has been 

 ascertained by experiment that they are not sufficient, as has been supposed, to maintain the 

 eyeballs in a fixed position during inclined movements of the head. 



In these different movements the six muscles of the eyeball may be considered as acting in 

 three pairs. 1st. In the horizontal movements the internal and external recti muscles are the 

 sole agents, the one acting as an adductor and the other as an abductor ; and this movement 



