ORBIT. 



785 



the third, and the optic nerves, with the 

 ophthalmic artery and the eye-ball. 



The obliquns mipcrinr is a long and slender 

 muscle, which is sometimes called \hetrochlearis 

 muscle from the fact of it being reflected through 

 a trochlea or pulley. It arises from the fibrous 

 sheath of the optic nerve, and from the inner 

 part of the optic foramen between the superior 

 and internal recti muscles ; it passes forwards 

 along the internal superior angle of the orbit, in 

 the form of a rounded fleshy belly, to which 

 succeeds a rounded tendon, which after passing 

 through the pulley beneath the internal angular 

 process is directed backwards, outwards, and 

 downwards, passing beneath the superior rectus 

 muscle to be inserted by a thin aponeurosis into 

 the sclerotic coat between the superior and exter- 

 nal rectus, rather behind the anterior half of the 

 globe. This pulley or trochlea, through which 

 passes the tendon of the superior oblique, is a 

 small cartiliginous ring, inserted by means of 

 fibrous tissue into a depression beneath the 

 internal angular process ; it is lined by a syno- 

 vial membrane. The orbital surface of the 

 superior oblique is in contact with the perios- 

 teum ; the fourth nerve passes into this surface 

 about its centre; the relations of its ocular 

 surface are the same as those of the superior 

 rectus. 



The superior rectus and superior oblique 

 muscles may now be cut through and turned 

 aside, and after removing carefully some fat 

 and cellular tissue the following parts are 

 brought into view : the internal and external 

 recti muscles, the optic, the third, and the nasal 

 branch of the ophthalmic nerves, the lenticular 

 ganglion between the optic nerve and external 

 rectus muscle, and the ophthalmic artery and 

 vein. 



The third nerve before entering the orbit 

 divides into two portions ; a superior smaller, 

 and an inferior larger; it enters the orbit through 

 the sphenoidal fissure between the two heads of 

 the external rectus muscle ; the inferior division 

 then passes beneath the globe of the eye and 

 must be examined in a subsequent stage of the 

 dissection. The superior division is now visible; 

 it passes to the under surface of the superior 

 rectus muscle, to which it sends numerous fila- 

 ments ; some filaments also pass on the inner 

 side of the superior rectus and enter the deep 

 surface of the levator palpebras superioris; these 

 are the only muscles supplied by this division 

 of the third nerve. 



The nasal nerve is in size the second branch 

 of the first division of the fifth. It enters the 

 orbit through the sphenoidal fissure, passing 

 between the two heads of the external rectus 

 muscle, in company with the third and sixth 

 nerves, being external to the former, and be- 

 tween its two divisions, and internal and some- 

 what superior to the latter. Having entered the 

 orbit, it passes forwards and inwards towards the 

 internal wall, crossing over the optic nerve 

 between it and the superior rectus muscle; it 

 has also above it the levator palpebra and supe- 

 rior oblique muscles, and the superior division 

 of the third nerve ; it passes out of the orbit 

 through the anterior internal orbital hole in 



VOL. III. 



company with the anterior ethmoidal artery. 

 Within the orbit it sends off lenticular, ciliary, 

 and infra-trochleur branches. The lenticular 

 branch is given off on the outer side of the optic 

 nerve ; it anastomoses with the superior division 

 of the third nerve, and joins the posterior supe- 

 rior angle of the lenticular ganglion. The 

 ciliary branches are two or three in number ; 

 given off above the optic nerve, they pass for- 

 wards and pierce the posterior part of the 

 sclerotic. The infra-trochlear branch is given 

 off near the inner wall of the orbit ; it passes out 

 beneath the pulley of the superior oblique 

 muscle, and sends branches to the superior eye- 

 lid, the lachrymal sac, and integuments of the 

 nose ; within the orbit there is an anastomosis be- 

 tween this and the supra-trochlear or frontal nerve. 



The lenticular or ciliary ganglion is situated 

 in the posterior and outer part of the orbit 

 between the optic nerve and the external rectus 

 muscle; it is very small, and of a somewhat 

 square form. Its superior posterior angle is 

 joined by the lenticular branch of the nasal nerve, 

 which constitutes the long root of the ganglion ; 

 its inferior posterior angle receives a branch 

 from the inferior division of the third nerve ; this 

 forms the short root of the ganglion. To its 

 posterior part is also connected one filament 

 from the cavernous plexus, and occasionally 

 one from the spheno-palatine ganglion. From 

 the anterior part of the ganglion a number of 

 delicate ciliary nerves pass or! ; they are divided 

 into two sets, one set coming from the superior 

 anterior, and the other from the inferior anterior 

 angle of the ganglion ; the former are the more 

 numerous; in all they are from twelve to sixteen 

 in number; they pass forwavds and pierce the 

 sclerotic near the optic nerve. 



The optic nerve passes forwards from the 

 optic hole to the inner and back part of the 

 eyeball, which it enters to terminate in the re- 

 tina. It is invested by a sheath of fibrous 

 membrane, which is continuous behind with the 

 dura mater, and in front with the sclerotic ; the 

 outer surface of this sheath posteriorly gives 

 attachment to the recti muscles, which surround 

 the optic nerve as it emerges from the optic 

 hole. The optic nerve is crossed above by the 

 nasal nerve and the ophthalmic artery, below by 

 the branch of the inferior division of the third 

 nerve, which supplies the internal rectus muscle; 

 it is surrounded by numerous delicate ciliary 

 nerves and arteries. 



The ophthalmic artery passes through the 

 optic hole in company with the optic nerve, 

 and inclosed in a sheath derived from the dura, 

 mater. It is very tortuous and twines round 

 the optic nerve, being at first inferior to the 

 nerve, then passing to its outer side, and soon 

 crossing over it to reach its inner side ; it then 

 passes across to the inner wall of the orbit, 

 where it breaks up into its terminal branches. 

 The branches of the ophthalmic artery are very 

 numerous ; they may be arranged in three sets ; 

 the first set arises from the artery as it lies ex- 

 ternal to the optic nerve; it consists of the 

 lachrymal and the centralis retinae ; the second 

 set comes off from the artery, when it is above 

 the optic nerve ; this consists of the supra- 



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