SECT. VI.-DISSECTION OF THE APPENDAGES OF THE EYE WITHIN THE ORBIT.

To dissect the appendages of the eye within the orbit, the orbital plate of the frontal bone must first be cut away. This can easily be done, after the calvaria has been removed, with a small sharp chisel or saw. The former is preferable, because it can be accommodated to the inequalities of the surface which is to be cut. The lesser wing of the sphenoid may also be removed with the orbital plate, care being taken not to destroy the optic foramen, nor disturb the contents of the sphenoidal fissure. Before removing the orbital ridge, the tendinous loop for the passage of the tendon of the superior oblique muscle should be examined, or the inner extremity of the ridge to which the loop is attached may be left. In removing the orbital plate, the periosteum which covers its under surface, and to which it is loosely connected, should be preserved.

Before proceeding to examine the contents of the orbit, the student will do well to obtain some idea of what they consist, and in what part of the orbit each is to be found. The dissection must also be conducted slowly and cautiously, so as not to destroy any of the minuter structures involved. The upper plane of the orbit is occupied on the nasal side by the superior oblique muscle, with the pathetic nerve resting on it; in the central portion by the frontal nerve, and immediately below it, the levator palpebræ superioris; and in the outer part by the lachrymal nerve, and beneath it the upper margin of the external rectus, on the outer side of which lies the lachrymal gland.

The FRONTAL NERVE, Fig. 20, enters the orbit above the muscles, and proceeds along its central portion, close to the periosteum, towards the supra-orbital foramen, where it divides into the supra-orbital and frontal branches. The former sends filaments to the upper eyelid, and then passes through the supra-orbital foramen to the forehead; the latter gives off filaments to the eyelid, also one to the frontal sinus, and one which anastomoses with the external nasal; its terminal branches are distributed on the forehead.

Fig. 20.

THE NERVES IN THE ORBIT ABOVE THE MUSCLES, BROUGHT INTO VIEW BY REMOVING THE ROOF OF THE ORBIT AND

3.

THE PERIOSTEUM.-1. Fifth nerve. 2. Ophthalmic branch of same nerve. Third nerve. 4. Fourth nerve. 5. Optic nerve. 6. Sixth nerve. a. Internal carotid artery.

the roof of the orbit in front of the optic foramen. It widens as it extends forwards, and is inserted by a broad tendon into the upper margin of the superior tarsal cartilage.

The SUPERIOR OBLIQUE MUSCLE, Fig. 23 (5), arises from the inner part of the optic foramen, passes along the inner and upper part of the orbit to the cartilaginous pulley, through which its tendon passes to be reflected downwards, backwards, and outwards to the ball of the eye. The reflected portion of this muscle must be left for the present.

The FOURTH NERVE, or the PATHETIC, Fig. 20 (4), enters the orbit above, and on the inner side of the frontal, which it leaves in the posterior part of the orbit, passing inwards to the superior oblique muscle, which it penetrates by several filaments on the orbital side.

The LACHRYMAL NERVE, Fig. 37 (10), enters the orbit a little to the outside of, and below the frontal nerve, and above the muscles, runs along the upper border of the external rectus to the lachrymal gland, to which it sends filaments; after perforating or passing beneath the gland, it divides into palpebral branches. One of these anastomoses with a branch of the facial, one goes to the integument upon the forehead, and one or two filaments pass downwards to join the second division of the fifth pair.

These parts may now be divided in the anterior portion of the orbit, and reflected back. As the levator palpebræ is turned back, a branch of the third nerve, or motor oculi, will be seen entering it on its under surface. The nerves above described should all be preserved for the purpose of tracing them through the cavernous sinus.

The SUPERIOR RECTUS MUSCLE, Fig. 23 (9), is now seen in the central part of the orbit. It arises from the upper and outer part of the optic foramen, and passes forwards to be inserted into the sclerotic coat of the eyeball, near its junction with the cornea. This is to be divided near its insertion, and turned back, observing at the same time the branch of the motor oculi nerve, which penetrates its under surface. Beneath the superior rectus, and between the external and internal recti muscles, is the most intricate part of the anatomy of the orbit. The areolar and adipose tissue found here must be gradually and cautiously removed.

Fig. 21.

The NASAL NERVE, Fig. 21, with the ophthalmic artery and vein, crosses the optic nerve from without inwards. As this nerve enters the orbit between the two heads of the rectus externus, it usually sends a small twig to the lenticular ganglion, and just as it passes over the optic nerve, gives off three or four ciliary branches, which accompany that nerve on its inner side to perforate the sclerotic coat. The nasal nerve then continues its course to the inner part of the orbit, and forwards as far as the anterior ethmoidal foramen, where it divides into an external and internal nasal branch. The internal nasal passes through this foramen, and after running a short distance on the cribriform plate of the ethmoid bone, enters the nose through a small foramen at the side of the crista galli,

THE DEEP NERVES OF THE ORBIT SEEN FROM ABOVE BY REMOVING THE BONE AND

DIVIDING THE ELEVATOR OF THE UPPER

EYELID AND THE UPPER RECTUS MUSCLE. poral muscle. c. Cut surface of bone. d. -a. Internal pterygoid muscle. b. TemElevator of the eyelid and upper rectus muscle. e. Carotid artery. 1. Optic nerve. 2. Fifth nerve. 3. Ophthalmic nerve. 4. Third nerve. 5. Sixth nerve.

to be distributed to the integument covering the ala of the

nose.

The external nasal, or infra-trochlear, goes to the anterior and inner part of the orbit, beneath the tendon of the superior oblique, where it divides into filaments to supply the integument of the upper part and side of the nose, the upper eyelid, and lachrymal sac.

The LENTICULAR GANGLION, Fig. 22 (8), is a small reddish body, situated between the optic nerve and external rectus muscle, about half an inch behind the ball of the eye. Its size and shape vary in different subjects. Behind, it receives a filament from the nasal and the lower division of the motor oculi, and is usually connected with the sympathetic by a branch from the cavernous sinus. Anteriorly it sends off eight or ten ciliary branches, which pass along the outer and under part of the optic nerve to perforate the sclerotic coat. This small ganglion may readily be found by tracing one of the ciliary nerves backwards to its origin.

The OPHTHALMIC ARTERY, a branch of the internal carotid, arises near the anterior clinoid process, and enters the orbit through the optic foramen, behind and exterior to the optic nerve, and crosses it to reach the nasal nerve, which it accompanies to the inner and anterior part of the orbit, where it divides into its terminating branches. The following are branches of this artery:

The lachrymal branch arises between the superior and external recti muscles, but soon joins the lachrymal nerve, and accompanies it to the lachrymal gland, which it supplies. It also sends branches to the upper eyelid, to the conjunctiva, and one which perforates the malar bone to anastomose with the deep temporal arteries. It sometimes anastomoses with the middle meningeal.

The central artery of the retina penetrates the optic nerve about half an inch behind the eyeball, which it enters through the porus opticus, to be distributed to the retina and hyaloid membrane (Fig. 21).

The supra-orbital arises beneath the superior rectus, and passes upwards and forwards on the inner side of this muscle and the levator palpebræ, joins the supra-orbital nerve, and, with it, goes to the forehead, sending a small branch to the frontal bone.

The ciliary arteries consist of three sets; the anterior, which are irregular in their origin, perforate the sclerotic coat about two lines behind the margin of the cornea; the long and

A VIEW OF THE THIRD, FOURTH, AND SIXTH PAIRS OF NERVES.-1. Ball of the eye, the rectus externus muscle being cut and hanging down from its origin. 2. The superior maxilla. 3. The third pair, or motor oculi, distributed to all the muscles of the eye except the superior oblique and external rectus. 4. The fourth pair, or pathetic, going to the superior oblique muscle. 5. One of the branches of the fifth. 6. The sixth pair, or motor externus, distributed to the external rectus muscle. 7. Spheno-palatine ganglion and branches. 8. Ciliary nerves from the lenticular ganglion, the short root of which is seen to connect it with the third pair.

short posterior arise a little distance behind the ball of the eye; the short, ten or fifteen in number, accompany the ciliary nerves; the two long ones perforate the sclerotic coat, one on either side of the optic nerve, and a little farther from it than the short.

The muscular branches are divided into the superior, which go to the superior rectus, oblique, and levator palpebræ; and the inferior, which are distributed to the other recti muscles, and inferior oblique, and from which are derived some of the anterior ciliary arteries.

The ethmoidal branches pass through the anterior and posterior ethmoidal foramina, and give branches to the dura mater and upper part of the nasal fossa.

There are two palpebral branches; the superior, which enters the upper lid near the inner angle and anastomoses with the