170 OCULAR GROUP. 



thinness of the orbital plate of the frontal bone and lesser wing of the 

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

 forwards by a smart blow,* and the external angular process and ex- 

 ternal 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 periosteum 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 

 ^<fsheath 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 inserted 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 palpebrae muscle ; 

 by the wider 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 sur- 

 face 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 pe- 

 riosteum of the orbit ; and by its upper border with the anterior and 

 posterior ethmoidal vessels, the nasal and supra-trochlear nerve. 



