RECTI OBLIQUI. 149 



rior oblique branch of the third nerve, the adipose tissue of the 

 orbit, and the under surface of the globe of the eye. By its under 

 surface with the periosteum of the floor of the orbit, and with the 

 inferior oblique muscle. 



The rectus internus (adductor), the thickest and shortest of the 

 straight muscles, arises from the common tendon, and from the 

 fibrous sheath of the optic nerve ; and is inserted into the inner 

 surface of the globe of the eye at two lines from the margin of the 

 cornea. 



Relations. By its internal surface with the optic nerve, the adi- 

 pose tissue of the orbit and the eyeball. By its outer surface with 

 the periosteum of the orbit ; and by its upper border with the ante- 

 rior and posterior ethmoidal vessels, the nasal and supra-trochlear 

 nerve. 



The rectus externus (abductor), the longest of the straight mus- 

 cles, 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 

 periosteum of the orbit ; and by the upper border with the lach- 

 rymal vessels and nerve and the lachrymal gland. 



The recti muscles present several characters which are common 

 to all : thus, they are thin; have 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 

 circumference of the cornea. The points of difference relate to 

 thickness and length ; the internal rectus is the thickest and most 

 short, 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 beneath the superior rectus muscle, to the outer and poste- 

 rior 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 

 inferior surface with the adipose tissue of the orbit, the upper 



