640 



DISSECTION OF THE ORBIT. 



chisel, 



and bone 

 forceps. 



Periosteum 

 of orbit. 



Open 

 periosteum. 



Position of 



parts 



exposed. 



Trace super- 

 ficial nerves, 



Orbit has 

 seven 

 muscles : 



the following manner. Two cuts are to be made with the saw through 

 the frontal bone, the inner one vertically over the internal margin of 

 the anterior opening of the orbit, and the outer one, commencing 

 behind the temporal crest, obliquely downwards and inwards, to 

 the external angular process : then with a chisel these are to be 

 continued backwards along the roof of the orbit, so as to meet near 

 the optic foramen. The piece of bone included between the incisions 

 is now to be tilted forwards, but is not to be taken away. This can 

 be done by knocking forwards the piece of frontal bone between the 

 saw-cuts with a mallet, and the orbital plate of the bone will be 

 carried upwards from the periosteum beneath. 



Afterwards the rest of the roof of the orbit, which is formed by 

 the small wing of the sphenoid bone, is to be cut away with the 

 bone forceps, except a narrow ring around the optic foramen ; and 

 any overhanging bone, which may interfere with the dissection, 

 should be likewise removed. During the examination of the cavity 

 the eye is to be pulled gently forwards. 



The periosteum of the orbit is now seen where it has been detached 

 from the bone in the dissection. This membrane forms a sac around 

 the contents of the orbit which is continuous posteriorly with the 

 dura mater through the sphenoidal fissure and the optic foramen, and 

 is closed in front by the palpebral fascia passing from it to the lids. 

 It adheres but loosely to the bones, and is perforated behind by 

 apertures for the passage of the vessels and nerves entering the orbit. 

 On the sides, prolongations of the membrane accompany the vessels 

 and nerves leaving the cavity. 



Dissection. The periosteum is next to be divided along the 

 middle of the orbit, and to be taken away. After the removal of a 

 little fat, the following nerves, vessels, and muscles come into view 

 (fig. 227, p. 642) ; but it is not needful to remove much of the fat at 

 this stage of the dissection. 



The frontal nerve and the supraorbital artery are placed in the 

 centre ; the lachrymal nerve and vessels close to the outer wall ; 

 and the small fourth nerve at the back of the orbit : all these nerves 

 are above the muscles in the cavity. The superior oblique muscle 

 lies on the inner side, and is recognised by the fourth nerve entering 

 its upper aspect ; the levator palpebrce and superior rectus are beneath 

 the frontal nerve ; and the external rectus is partly seen below the 

 lachrymal nerve. At the outer part of the orbit, near the front, is 

 the lachrymal gland. 



The frontal and lachrymal nerves should be followed forwards 

 to their exit from the orbit, and backwards, with the fourth 

 nerve, through the sphenoidal fissure, to the wall of the cavernous 

 sinus. In tracing them back, it will be expedient to remove the 

 projecting anterior clinoid process, should this still remain ; and 

 some care will be required to follow the lachrymal nerve to its 

 commencement. 



Contents of the orbit. The eyeball, the lachrymal gland, and a 

 quantity of granular fat, are lodged in the orbit. Connected with 



