208 THE HEAD AND NECK 



muscle. When it is paralysed, the medial rectus is unopposed 

 and a convergent strabismus results. In an endeavour to com- 

 pensate for this disability, the patient carries his head rotated 

 to the affected side. 



In the intracranial part of their course the motor nerves of 

 the orbit may be pressed on by the exudate of a basal meningitis 

 (p. 225), by haemorrhage following fractures of the base of the 

 skull, or by thrombosis of the cavernous sinus as they lie in its 

 lateral wall (p. 225). 



Tenotomy of the Ocular Tendons is performed in cases of 

 strabismus which are due to congenital shortening of the medial 

 or lateral rectus muscles. An incision is made through the 

 conjunctiva near the margin of the cornea, and the underlying 

 fascia bulbi (capsule of Tenon) is opened. A blunt hook may be 

 inserted through the wound, and the desired tendon can be 

 drawn forwards and divided. 



The Ophthalmic Artery is a branch of the internal carotid. It 

 supplies the muscles of the orbit, the lacrimal gland, and the 

 eyelids. Ciliary branches supply the eyeball, and the arteria 

 centralis retinae, which runs forwards in the optic nerve, is 

 distributed to the retina. 



The Ophthalmic Veins communicate with the pterygoid 

 venous plexus through the inferior orbital (spheno-maxillary) 

 fissure, and with the anterior facial vein (p. 175). They pass 

 backwards through the superior orbital (sphenoidal) fissure and 

 join the cavernous sinus. 



The Eyeball. The Sclera, a strong fibrous membrane, 

 forms the outermost coat of the eye. Anteriorly, it lies deep to 

 the conjunctiva and the fascia bulbi and blends with the cornea. 

 It is supplied by branches from the ciliary arteries, and as these 

 vessels approach the corneo-scleral junction they send out 

 branches into the episcleral tissue, which form anastomosing 

 loops close to the margin of the cornea. From these loops 

 numerous small straight branches pass into the periphery of the 

 cornea, where they anastomose with one another. In inflam- 

 mation of the sclera these vessels become visible, and form the 

 " zone of ciliary injection." 



The Cornea consists mainly of modified connective tissue. 

 Except at its extreme edge, it is non-vascular, but it is well 

 provided with spaces containing lymph, on which it depends for 

 its nutrition. The lymph is drained from the cornea by the 

 sinus venosus sclerce (canal of Schlemni). 



