The palpebral arterie.s, two in number, arise near the inner angle of the orbit. The superior arises above the tendon of the orbicularis palpe brarum ; it passes outwards and supplies th upper lid, one branch running near the 'tarsal margin of the lid, vvhile the others are distri buted to the muscles and integ,uments of the middle and upper part of the lid, where they anastomose with the supra-orbital artery. The • inferior palpebral artery passes down behind the tendon of the orbicularis, then runs out wards along the lower lid, forming an ar near the free margin of the lid, and is gradual lost near the external canthus. It anastomo with the angular branch of the facial, with t infra-orbital braneh of the internal maxilla vvith the trinsverse facial and temporal art Beneath the internal angular process th thalnriic artery terminates by dividing in na.sal arid theirontal bmnches.
The .nasal artery emerges from the above the tendon of the orbicularis ; it , moses freely with the angular artery, branches to the lachrymal sac, and ten in a branch which passes down the dors the nose, and communicates at the ext of the nose with the corresponding ar the opposite side.
The frontal artery passes out pf th with the nasal, then turns upwards, and tributed to the muscles and integuments forehead, anastomosing, with the supra and with the arteries of the opposite side.
The ophthalmic vein commences at the i angle of the orbit, where it communicates fr with the angular and frontal veins; it p 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 ciliar'y 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 siath 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 water. 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 iVerior division qf 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 alinost 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 bmnch which goes to the infdior 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 ttvo 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 1 of the orbit; it turns over the globe of the eye, and is inserted by a thin tendinous expan ision just behind the margin of the cornea. A Ismail bursa intervenes between the tendon and the sclerotic, as is the case with the tendons of loll the recti muscles.
The internal rectus arises from the common endon or ligament of Zinn, and from the 1 brous 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.