1034 



ORGANS OF SPECIAL SENSE 



muscles, and is in greatest amount immediately behind the eyeball; it also exists 

 between the muscles and the orbital walls in the anterior half ot the cavity. Six 

 muscles, viz., the four recti, the superior oblique, and the levator palpebrse supe- 

 rioris, arise at the apex of the orbit, and diverge as they pass forwards. The recti 

 muscles superior, inferior, external, and internal run each near the correspond- 

 ing orbital wall, but the superior is overlapped in part by the levator palpebrse. 

 The superior oblique lies about midway between the superior and internal recti. 

 A seventh muscle, the inferior oblique, has a short course entirely in the anterior 

 part of the orbit, coming from its inner wall and passing beneath the globe between 

 the termination of the inferior rectus and the orbital floor. The optic nerve with 

 its sheaths passes from the optic foramen to the back of the eyeball, surrounded by 

 the orbital fat, and more immediately by a loose connective tissue. Among the 

 contents of the cavity are also to be enumerated many vessels and nerves and 

 fibrous tissue septa, while its walls are clothed by periosteum (periorbita). 



The muscles of the orbit are seven in number, of which six are ocular, i. e., are 

 inserted into the eyeball and rotate it in different directions. These ocular muscles 

 are arranged in opponent pairs, viz., superior and inferior recti, superior and 

 inferior obliques, external and internal recti. With the exception of the short 



FIG. 732. LEFT EYEBALL SEEN IN ITS NORMAL POSITION IN THE ORBIT, WITH VIEW 

 OF THE OCULAR MUSCLES. (After Merkel, modified.) 

 > 



Tendon of superior oblique 



Levator palpebrse super- 



ioria, cut 

 Superior rectua 



Internal reetus 



Inferior oblique muscle 



External rectua 



Inferior rectus 



inferior oblique, they all arise from the back of the orbit along with the seventh 

 orbital muscle, the levator palpebrse superioris. All these long muscles take their 

 origin from the periosteum in the vicinity of the optic foramen. The four recti 

 muscles arise from a fibrous ring : v the annulus tendineus communis (ligament 

 of Zinn), which arches close 'Over the upper and inner edge of the foramen, and 

 extends down and out so as to embrace part of the opening of the superior orbital 

 (sphenoidal) fissure. Their origins may be said at first to form a short, common, 

 tendinous tube, from which the individual muscles soon separate, taking the positions 

 indicated by their respective names. The external rectus has two origins from bone, 

 one on either side of the superior orbital fissure. But in the 'fresh state the fissure 

 is here bridged across by fibrous tissue, from which this rectus also springs, so 

 that its origin is in reality continuous. The part of this fibrous ring nearest the 

 foramen (corresponding to the origins of the superior and internal recti) is qjosely 

 connected with the outer sheath of the optic nerve. The remaining two long mus- 

 cles arise just outside the upper and inner part of the above-mentioned ring, and 

 are often partially united; the levator palpebrse tendon is in close relation to the 

 origin of the superior rectus, while the superior oblique arises from the periosteum 



