ti52 A MANUAL OF ANATOMY 



lachrymal bone, in association with the foi"ward expansions from 

 the sheaths of the external and internal recti muscles. It is ex- 

 panded beneath the eyeball, to the fore part of which it acts as a 

 sling or support. 



The muscular sheaths are derived from the capsule of Tenon. 

 The tendons of the six ocular muscles pierce the capsule on their 

 way to their insertions, which latter are under cover of the capsule, 

 each muscle receiving a prolongation from the margin of the cleft 

 in the capsule of Tenon through which its tendon passes. These 

 prolongations extend backwards around the muscles, and ultimately 

 blend with their perimysial sheaths. As regards the superior oblique 

 muscle, the prolongation around its tendon extends upwards, 

 inwards, and forwards as far as the margin of the trochlea, to 

 which it is attached. The prolongation around the tendon of the 

 inferior oblique muscle extends downwards as far as the outer 

 part of the floor of the orbit. 



The sheaths of the four recti muscles give off important expan- 

 sions. The expansion from the sheath of the external rectus is 

 strong, and is attached to the orbital process of the malar bone, 

 and that from the sheath of the internal rectus, also strong, is 

 attached to the lachrymal crest of the lachrymal bone. These two 

 expansions, at their bony attachments, are connected with the 

 lateral extremities of Lockwood's suspensory ligament. The ex- 

 pansion from the sheath of the superior rectus blends with the 

 deep layer of the tendon of the levator palpebrae superioris, and 

 that from the sheath of the inferior rectus is connected with the 

 inferior tarsal plate. These expansions from the sheaths of the 

 recti, especially from those of the external and internal recti, 

 moderate the action of the muscles, and the latter two are hence 

 known as the check ligaments. 



Nerves in the Orbit — Optic Nerve. — ^The optic nerve extends for- 

 wards and outwards from the optic commissure, and enters the 

 orbit through the optic foramen, having the ophthalmic artery 

 below it at first, and then on its outer side. It receives sheaths 

 from the dura mater and arachnoid, the former being strong, and 

 at its entrance into the orbit it is surrounded by the origins of the 

 four recti muscles. Its direction is forwards and outwards, with 

 a slight inclination downwards, to the back part of the eyeball, 

 where it pierces the sclerotic coat about ^ inch internal to, and a 

 little below, the level of the centre. Thereafter it pierces the 

 choroid coat, and terminates in the nerve fibre-layer of the retina. 

 The lenticular or ciliary ganglion is in close contact with the 

 outer side of the nerve towards the back part of the orbit, and 

 anterior to this the nerve is surrounded by the ciliary nerves and 

 vessels. It is crossed superiorly by the ophthadmic artery, the 

 superior ophthalmic vein, and the nasal nerve. Inferiorly, about 

 J inch behind the eyeball, the arteria centralis retina? enters it as 

 far as the centre, and thereafter runs forward within it to the 

 retina. 



