THE ORBIT AND ITS FASCIAE. 



H37 



with the optic foramen. The median walls of the two orbits are slightly divergent 

 behind, but almost parallel with the sagittal plane and with each other ; the lateral 

 walls are obliquely placed and with the sagittal plane form an angle of about 48 

 and, therefore, with each other one of something more than a right angle. The axis 

 of the orbit is directed inward and upward, forming an angle of from i5-2o with 

 the horizontal plane, and one of about 45 with the orbital axis of the opposite side, 

 which it intersects in the vicinity of the sella turcica. The width of the orbital en- 

 trance is about 4 cm. and the height about 5 mm. less, while the depth of the orbit is 

 approximately 4 cm. The space, therefore, is much more capacious than necessary 

 to accomodate the eyeball and the associated muscles, blood-vessels and nerves. 

 The interspaces thus left are occupied by the orbital fat (corpus adiposum orbitae), sup- 

 ported by a framework of connective tissue lamellae prolonged from the orbital fascia 

 which, in turn, is continuous with the periosteum lining the orbit. The latter, also 

 known as the periorbita, is thin but resistant and at the various openings in the walls of 

 the orbit continuous with the periosteum covering the adjacent surfaces of the skull. 



FIG. 1198. 



Eyelid 



Nasal fossa, 

 Anterior ethmoi 



Mesial orbital wall, 



Internal rectus muscle 

 Posterior ethmoidal cells 



Conjunctival sac 

 Anterior chamber 

 Cornea 

 Lens 



Vitreous chamber 



Check ligament 

 Orbital wall 



External rectus muscle 



Sclera 



Orbital fat 

 Optic nerve- 

 Horizontal section of right orbit showing eye in position. 



The eyeball does not rest directly in contact with the fatty cushion forming the 

 walls of the cup-shaped recess in which it lies, but is separated from the surrounding 

 adipose tissue by a fascial investment, the capsule of Tenon (page 504). This 

 sheet covers the posterior three-fourths of the eyeball and encloses, between it and 

 the eye, the space of Tenon. The latter in front begins beneath the conjuctival 

 sac, close to the corneal margin, and behind ends in the vicinity of the optic nerve. 

 It does not, however, quite reach the latter, but terminates where the eyeball is 

 pierced by the posterior ciliary vessels and nerves, thus leaving an irregular oval area 

 uncovered (Merkel). Farther backward the space of Tenon communicates with the 

 subdural lymph-channel prolonged along the optic nerve and thus establishes relations 

 with the intracranial lymph-paths (page 949). 



The eye muscles, which together with the elevator of the upper lid have been 

 described (page 502), are invested by fascial sheaths prolonged from the orbital 

 periosteum. These sheaths increase in thickness as they approach the eyeball until, 

 at the points where the tendons of the ocular muscles meet the fascial sheet investing 

 the posterior part of the eye the capsule of Tenon the muscle sheaths blend with 

 this capsule on the one hand, and, on the other, are attached at certain points to the 

 orbital wall as robust pointed processes of considerable strength. One such process, 



