OCULAR GROUP. 171 



The RECTUS EXTERN us (abductor), the longest of the straight 

 muscles, arises by two distinct heads, one from the common tendon, 

 the other with the origin of the superior rectus from the margin of 

 the optic foramen ; the nasal, third and sixth nerves passing between ' 

 its heads. It is inserted into the outer surface of the globe of the eye 

 at a little more than two lines from the margin of the cornea. 



Relations. By its internal surface with the third, the nasal, the 

 sixth, and the optic nerve, the ciliary ganglion and nerves, the oph- 

 thalmic artery and vein, the adipose tissue of the orbit, the inferior 

 oblique muscle, and the eyeball. By its external surface with the pe- 

 riosteum of the orbit ; and by the upper border with the lachrymal 

 vessels and nerve and the lachrymal gland. 



The recti muscles present several characters which are common to 

 all ; thus, they are thin, have each the form of an isosceles triangle, 

 bear the same relation to the globe of the eye, and are inserted in a 

 similar manner into the sclerotica, at about two lines from the circum- 

 ference of the cornea. The points of difference relate to thickness and 

 length ; the internal rectus is the thickest and shortest, the external 

 rectus the longest of the four, and the superior rectus the most thin. 

 The insertion of the four recti muscles into the globe of the eye forms 

 a tendinous expansion, which is continued as far as the margin of the 

 cornea, and is called the tunica albuginea. 



The OBLIQUUS SUPERIOR (trochlearis) is a fusiform muscle arising 

 from the margin of the optic foramen, and from the fibrous sheath of 

 the optic nerve ; it passes forwards to the pulley beneath the internal 

 angular process of the frontal bone ; its tendon is then reflected be- 

 neath the superior rectus muscle, to the outer and posterior part of the 

 globe of the eye, where it is inserted into the sclerotic coat, near the 

 entrance of the optic nerve. The tendon is surrounded by a synovial 

 membrane, while passing through the cartilaginous pulley. 



Relations. By its superior surface with the fourth nerve, the supra- 

 trochlear nerve, and with the periosteum of the orbit. By the in- 

 ferior surface with the adipose tissue of the orbit, the upper border of 

 the internal rectus and the vessels and nerves in relation with that 

 border. 



The OBLIQUUS INFERIOR, a thin and narrow muscle, arises from 

 the inner margin of the superior maxillary bone, immediately external 

 to the lachrymal groove, and passes beneath the inferior rectus, to " 

 inserted into the outer and posterior part of the eyeball, at about two 

 lines from the entrance of the optic nerve. 



Relations. By its superior surface with the inferior rectus muscle 

 and with the eyeball ; and by the inferior surface with the perios- 

 teum of the floor of the orbit, and the external rectus muscle. 



According to Mr. Ferrall* the muscles of the orbit are sepa 

 from the globe of the eyeball and from the structures immediately 

 surrounding the optic nerve, by a distinct fascia, which is continuous 



* In a paper read before the Royal Society, on the 10th of June, 1841. 



