Periosteum of Orbit jj 



bone, and so into the nasal fossa. Below the floor of the orbit is the 

 antrum, and tumours from that region readily bulge into the orbit. Im- 

 mediately beneath the floor runs the superior maxillary division of the 

 fifth nerve (p. 62). 



Through the inner part of the floor descends the nasal duct, close 

 to the outer side of which arises the inferior oblique. On the roof is a 

 depression, at the outer part, for the lachrymal gland, and at the inner 

 part is lodged the pulley of the superior oblique. 



A large mass of yellow fat fills the back of the orbit, and forms a 

 soft bed against which the eyeball rests. In phthisis and other wast- 

 ing diseases this store of hydro-carbons is drawn upon and the eye 

 becomes sunken. 



A periostitis, an erysipelas, or other inflammatory condition of or 

 about the orbit may spread by direct continuity of tissue through the 

 optic foramen or the sphenoidal fissure into the interior of the skull, 

 and there give rise to meningitis or to intracranial suppuration. 

 In the case, moreover, of septic phlebitis in the ophthalmic vein the 

 clot may extend into the cavernous sinus and set up fatal thrombosis. 



The periosteum of the orbit is continuous through the optic foramen 

 and the sphenoidal fissure with the dura mater ; and anteriorly it 

 turns round to spread into the pericranium. As the fibrous offshoot 

 from the dura mater enters through the optic foramen to line the 

 orbit it gives a tubular investment along the optic nerve, which, 

 spreading out upon the sclerotic, is ultimately reflected from the antero- 

 lateral part of the eye-ball, behind the conjunctiva, to the margin of 

 the orbit, where it blends with the periosteum. By this arrangement of 

 the fascia the eye-ball is completely shut off from the back of the orbit. 



A point of great surgical interest in connection with these fibrous 

 layers is that after removal of the superior maxilla (p. 18) the lower 

 periosteum becomes thickened and strengthened, and forms so useful 

 a floor to the orbit that there is but little permanent dropping of the 

 eye-ball ; the double vision which results from the first sinking of the 

 globe soon passes away. 



The capsule of Tenon is that part of the orbital fascia which sur- 

 rounds the optic nerve and eventually spreads round the eye-ball. 

 It has already been described as sending a post-conjunctival offshoot 

 to the periosteum of the orbit, but, in addition, it sends back fibrous 

 sheaths around the muscles of the eye-ball, which are intimately 

 joined with their proper fascial investments. 



The capsule of Tenon is connected with the sclerotic by delicate 

 filamentous tissue, and forms a smooth bed in which the globe moves 

 with absolute freedom. It is lined with endothelium, and is, in reality, 

 the outer wall of a large lymph-space, like the pleura or peritoneum. 

 The choroidal lymphatics enter the space around the venae vorticosae, 

 and the space itself is in communication, under the fibrous sheath of the 

 optic nerve, with the subdural and subarachnoid areas of the cranium. 



