ORBIT. 



787 



backwards in the same direction as the artery, 

 but it is much less tortuous. It receives 

 branches corresponding to those which the 

 artery gives off, and passing between the two 

 heads of the external rectus muscle below the 

 nerves, it terminates in the anterior extremity 

 of the cavernous sinus. 



The ophthalmic artery and vein may now be 

 removed ; cut through the optic and ciliary 

 nerves and remove some fat and cellular tissue 

 which obscures the remaining muscles and 

 nerves ; we now obtain a more clear view of the 

 internal and external recti muscles, and at the 

 same time we expose the sixth nerve and the 

 inferior division of the third, as well as the 

 inferior rectus and the inferior oblique muscles. 

 The sixth nerve passes between the two 

 heads of the external rectus muscle below the 

 third nerve, and above the ophthalmic vein, 

 from which it is separated by a process of dura 

 mater. Having entered the orbit it passes along 

 the inner surface of the external rectus muscle, 

 to which it is distributed by numerous delicate 

 filaments. 



The inferior division of the third nerve 

 enters the orbit, as we have seen, between the 

 two heads of the rectus muscle, where it lies a 

 little above the sixth nerve ; having entered the 

 orbit it passes down towards the floor between 

 the optic and the sixth nerves, and below the 

 level of the latter. It almost immediately di- 

 vides into three branches : an internal, which 

 passes inwards beneath the optic nerve towards 

 the internal rectus muscle, to the ocular surface 

 of which it is distributed ; a middle branch, 

 which is distributed in the same manner to the 

 ocular surface of the inferior rectus; and an 

 external, which passes forwards along the ex- 

 ternal border of the inferior rectus, and enters 

 the posterior border of the inferior oblique, 

 almost at right angles. The short filament 

 which joins the posterior inferior angle of the 

 lenticular ganglion, forming the short root of 

 the ganglion, is usually given off from the 

 branch which goes to the inferior oblique 

 muscle. 



The external rectus muscle has two origins, 

 one from a tendon, the tendon or ligament of 

 Zinn, which is common to this muscle with 

 the inferior and internal recti, and which is 

 attached to a little tubercle behind the optic 

 foramen; the other origin of the external rectus 

 is above, from the inner margin of the sphe- 

 noidal fissure ; this origin is united with the 

 origin of the superior rectus. Between these 

 two origins pass the third, the sixth, and the 

 nasal branch of the fifth nerves, with the oph- 

 thalmic vein. From its origin the external 

 rectus passes forwards along the external wall 

 of the orbit; it turns over the globe of the eye, 

 and is inserted by a thin tendinous expan- 

 sion just behind the margin of the cornea. A 

 small bursa intervenes between the tendon and 

 the sclerotic, as is the case with the tendons of 

 all the recti muscles. 



The internal rectus arises from the common 

 tendon or ligament of Zinn, and from the 

 fibrous sheath of the optic nerve ; it passes 

 forwards along the internal wall of the orbit, 



turns over the globe of the eye, and is inserted 

 immediately opposite the external rectus, in 

 the same manner as the other recti muscles. 



The inferior rectus muscle arises from the 

 common tendon, between the internal and ex- 

 ternal recti ; it passes forwards under the globe 

 of the eye and is inserted into the sclerotic in 

 the same manner as the preceding muscles, 

 and immediately opposite the superior rectus. 

 The recti muscles have all the same form, viz. 

 that of a long isosceles triangle, having the base 

 directed forwards, and the apex backwards. 

 They differ in length and thickness; the in- 

 ternal rectus being the shortest and thickest, 

 the external rectus the longest, and the superior 

 rectus the smallest.* 



The inferior oblique muscle is the only one 

 which does not arise from the apex of the orbit. 

 It arises from the orbital plate of the superior 

 maxillary bone, just within the margin of the 

 orbit, and near the groove for the lachrymal sac. 

 From its origin it passes obliquely outwards, 

 upwards, and backwards beneath the globe of 

 the eye and the inferior rectus, then between 

 the former and the external rectus ; it ends in 

 an aponeurotic expansion which is inserted 

 into the sclerotic between the superior and ex- 

 ternal recti, opposite the insertion of the su- 

 perior oblique, and rather nearer the optic 

 nerve than the insertion of that muscle. The 

 superior surface of this muscle is in contact 

 with the inferior rectus and the globe of the 

 eye ; the inferior touches the floor of the orbit 

 and the external rectus muscle; its borders are 

 anterior and posterior ; a branch of the third 

 nerve enters the posterior border. 



The orbital portion of the superior maxillary 

 nerve may now be exposed by cutting through 

 the external rectus muscle, and drawing the 

 eye with its muscles towards the inner part of 

 the orbit. The nerve having crossed the spheno- 

 maxillary fossa enters the orbit through the 

 spheno-maxillary fissure; in company with a 

 branch of the internal maxillary artery it passes 

 along the infra-orbital groove, covered fty a 

 layer of periosteum ; it then passes through the 

 canal and emerges from the infra-orbital fora- 

 men. The trunk of the nerve is but little vi- 

 sible on the floor of the orbit. While the supe- 

 rior maxillary nerve is in the foramen rotun- 

 dum, or during its passage across the fossa, it 

 sends off a temporo-malar branch which passes 

 through the spheno-maxillary fissure superior 

 and external to the trunk of the nerve; it passes 

 along the floor of the orbit, beneath the inferior 

 rectus muscle, and about the middle divides 

 into two branches, a temporal and a malar. 



The temporal branch goes towards the outer 

 wall of the orbit, passes up between the bone 

 and the external rectus muscle, and joins with 

 a temporal branch from the lachrymal ; it then 

 pierces the orbital process of the malar bone 

 and enters the temporal fossa, where it com- 

 municates with the anterior deep temporal 

 nerve, sends branches to the temporal muscle, 

 and piercing the fascia is distributed to the 

 skin over the temporal region. 



Cruveilhicr. 



Descriptive Anatomy. 

 3 E 2 



