114 TOPOGRAPHICAL ANATOMY OF THE 



orbit through the orbital fissure along with the ophthalmic and oculo- 

 motor. It divides into two branches which supply the lateral rectus and 

 retractor muscles. 



N. OPTICUS. — The optic nerve forms a conspicuous, rounded and 

 slightly flexuous cord surrounded by the retractor muscle aud extending 

 from the optic foramen, by which it emerges from the cranium, to the 

 back of the eyeball.^ The nerve pierces the sclera below and lateral to 

 the posterior pole of the eyeball. 



A. OPHTHALMICA. — The ophthalmic artery is a branch of the internal 

 maxillary, from which it arises within the alar canal. Leaving the 

 canal by its anterior opening, the artery at once enters the orbit (at the 

 apex of the cavity bounded by the periorbita), where it is related to the 

 naso-ciliary nerve, curves in a medial direction underneath the superior 

 rectus muscle, and is continued through the ethmoidal foramen as the 

 ethmoidal artery. 



The collateral branches of the ophthalmic artery are as follows : — 



(1) Ramii musculares. — These supply not only the muscles within 

 the orbit but also the periorbita, the fat of the orbit, the third eyelid 

 and the conjunctiva. 



(2) A. lacrimalis. — The lachrymal artery follow^s the like-named 

 nerve and terminates in the lachrymal gland and the upper eyelid. 



(3) Aa. ciliares anteriores. — The anterior ciliary arteries are slender 

 vessels that pierce the sclera in front of the equator of the eyeball and 

 terminate in the iris and the ciliary body. 



(4) Aa. ciliares iMsteriores. — The posterior ciliary arteries are also 

 small vessels. They penetrate the sclera at the back of the eyeball, and 

 most of them (the short ciliary arteries) end in the choroid. Two of 

 them (the long ciliary arteries), however, pass forwards between the 

 sclera and the choroid to end at the periphery of the iris. 



(5) A. centralis retince. — The central artery of the retina is a 

 slender vessel that may take origin from one of the posterior ciliary 

 arteries. It enters the optic nerve within which it gains the interior of 

 the eyeball to ramify over the surface of the posterior part of the 

 retina. 



(6) A. supraorhitalis. — The supraorbital artery follows the frontal 

 nerve along the medial wall of the orbit, traverses the supra-orbital 

 foramen, and is distributed in the muscles and skin of the supra-orbital 

 region, where it anastomoses with branches of the anterior auricular 

 artery. 



^ Strictly speaking the oi^tic nerve should be described as passing in the opposite 

 direction, for its fibres have their origin in the retina. 



