THE ORBIT 243 



removing the thick cranial wall above the orbital opening, 

 leaving only a thin portion corresponding to the superior orbital 

 arch. 



Strip the soft structures including the periosteum downwards 

 from the cut margin of the skull to the superior orbital margin. 

 Then take the saw and make two incisions through the frontal 

 bone to the floor of the anterior fossa. One, vertically down- 

 wards, opposite the anterior end of the cribriform plate of the 

 ethmoid, and the second downwards and forwards immediately 

 anterior to the lateral end of the small wing of the sphenoid. 

 After the saw cuts are made take the bone forceps and chip 

 away the frontal bone between the saw cuts, to the level of the 

 floor of the anterior fossa. As the bone is removed the frontal 

 air sinus may be opened and its extent should be noted (Figs. 

 89, 90). Next take the chisel and cut through the floor of 

 the anterior fossa immediately in front of the posterior border 

 of the small wing of the sphenoid. Carry the cut medially 

 to 2 mm. in front of the optic foramen ; then turn it forwards 

 along the medial border of the orbital plate of the frontal bone 

 and lateral to the depression in the region of the cribriform 

 plate of the ethmoid ; then, with the aid of the chisel and bone 

 forceps, gradually chip away the whole of the bony roof of the 

 orbit, but do not injure the subjacent periosteum. Next remove 

 the remains of the small wing of the sphenoid with the exception 

 of the margin of the optic foramen, which must be left intact. 

 The superior orbital fissure is now fully opened up, and no 

 difficulty will be met with as the nerves in the wall of the 

 cavernous sinus are traced forwards into the orbit, but the 

 anterior clinoid process may be chipped away to gain additional 

 freedom for further dissection. After the bony roof of the orbit 

 has been removed examine the exposed periosteum. 



Periosteum. If the dissection has been successfully 

 carried out, the periosteum clothing the under surface of the 

 orbital roof will be exposed uninjured. The periosteum of 

 the orbit forms a funnel-shaped sheath, which encloses all the 

 contents of the cavity except the zygomatic and infra-orbital 

 nerves and the infra-orbital artery. It is but loosely attached 

 to its bony walls. Posteriorly, it is directly continuous, 

 through the superior orbital fissure, with the dura mater. 

 Expanding with the cavity, it becomes continuous anteriorly, 

 around the orbital opening, with fhe periosteum which 

 clothes the exterior of 'the skull. There it presents im- 

 portant connections with the palpebral fascia also. 



Dissection. Open the eyelids and draw the eyeball forwards 

 with the forceps ; then, with \ fi.ne needle, carry a piece of 

 thread through the ocular conjunctiva, being careful not to 

 penetrate the eyeball, for that would render its subsequent 

 inflation impossible. Finally, stitch the thread to the nose, and 

 the eyeball will be securely held forwards. Turn now to the 

 periosteum of the roof of the orbit, divide it transversely, close 

 in 16 a 



