RECTUS EXTERNUS. 177 



sphenoid. The superciliary portion of the orbit may now be driven for- 

 wards by a smart blow, and the external angular process and external 

 wall of the orbit outwards in the same manner ; the broken fragments of 

 the roof of the orbit should then be removed. By this means the perios- 

 teum will be exposed unbroken and undisturbed. Remove the periosteum 

 from the whole of the upper surface of the exposed orbit, and examine the 

 parts beneath. 



The LEVATOR PALPEBR.E is a long, thin, and triangular muscle ; situated 

 in the upper part of the orbit on the middle line ; it arises from the upper 

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

 and is inserted into the upper border of the superior tarsal cartilage. 



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

 nerve and artery, the periosteum of the orbit, and in front with the inner 

 surface of the broad tarsal ligament. By its under surface it rests upon 

 the superior rectus muscle, and the globe of the eye ; it receives its nerve 

 and artery by this aspect, and in front is covered for a short distance by 

 the conjunctiva. 



The RECTUS SUPERIOR (attollens) arises from the upper margin of the 

 optic foramen, and from the fibrous sheath of the optic nerve, and is in- 

 serted into the upper surface of the globe of the eye at a point somewhat 

 more than three lines from the margin of the cornea. 



Relations. By its upper surface with the levator palpebrse muscle ; by 

 the under surface with the optic nerve, the ophthalmic artery and nasal 

 nerve, from which it is separated by a layer of fascia and by the adipose 

 tissue of the orbit, and in front with the globe of the eye, the tendon of the 

 superior oblique muscle being interposed. 



The RECTUS INFERIOR (depressor) arises from the inferior margin of the 

 optic foramen by a tendon (ligament of Zinn) which is common to it, the 

 internal and the external rectus, and from the fibrous sheath of the optic 

 nerve ; it is inserted into the inferior surface of the globe of the eye at a 

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



Relations. By its upper surface with the optic nerve, the inferior 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 adipose 

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

 teum of the orbit ; and by its upper border with the anterior and posterior 

 ethmoidal vessels, the nasal and supra-trochlear nerve. 



The RECTUS EXTERNUS (abductor), the longest of the straight muscles, 

 arises by two distinct heads, one from the common tendon, the other with 



muscle. 5. The superior oblique muscle. 6. Its cartilaginous pulley. 7. Its reflected 

 tendon. 8. The inferior oblique muscle ; the small square knob at its commencement 

 is a piece of its bony origin broken off. 9. The superior rectus. 10. The internal rectus 

 almost concealed by the optic nerve. 11. Part of the external rectus, showing its two 

 heads of origin. 12. The extremity of the external rectus at its insertion ; the inter 

 mediate portion of the muscle having been removed. 1 3. The inferior rectus. 14 

 The tunica albuginea, formed by the expansion of the tendons of the four reeti. 



M 



