PRACTICAL CONSIDERATIONS: ORBIT AND FASCIA. 1439 



is greatest just back of its anterior margin, which is thickened and offers the best 

 protection to the eye from injury. The upper margin is most marked and with the 

 eyebrow offers a good protection to the eye in that direction. The inner margin is 

 not prominent, but is well reinforced by the bridge of the nose. The outer edge is 

 least prominent, and on that side palpation is possible as far back as the equator of 

 the globe. For this reason, and because the outer walls converge backward while 

 the inner walls are parallel, incisions for reaching the interior of the orbit are best 

 made on the outer side. The walls are thin and easily fractured by direct violence, 

 as from canes and similar objects, which sometimes enter the adjacent cavities, as 

 the ethmoidal. Tumors may encroach upon the orbital space either by causing the 

 absorption of the thin intervening bone, or by growing through one or more of the 

 openings in its wall, as through the optic foramen and sphenoidal fissure from the 

 cranial cavity, the nasal duct from the nose, or the spheno-maxillary fissure from the 

 temporal or zygomatic fossae. 



The eyeball occupies about one-fifth of the orbital cavity, the remaining space 

 being filled by nerves, vessels, muscles, the lachrymal gland, fat, and a system of 

 fasciae. In the ordinary case a straight edge placed against the upper and lower 

 margins of the orbit will just touch the closed lids covering the apex of the cornea, 

 but will not compress the eye. A straight line between the two lateral margins 

 would pass back of the cornea, on the outer side posterior to the ora serrata and on 

 the inner side at the junction of the ciliary body and iris. 



An exophthalmos is a protrusion forward of the ball, and is usually due to 

 pressure from behind, more rarely to paralysis of the recti muscles. Some of the 

 more common causes of retrobulbar pressure are orbital cellulitis or abscess, tumors, 

 distension of the orbital vessels, and excess of fat. 



Enophthalmos, due to exhausting disease, is more apparent than real, but a true 

 sinking of the globe may be due to paralysis of Miiller's muscle due to lesion of the 

 sympathetic, to atrophy of the retro-bulbar cellular tissue caused by trophic dis- 

 turbance, to fracture and depression of the orbital bones with cicatricial adhesion 

 and contraction, and to injury of Tenon's capsule and the check ligaments. 



Inflammatioyi of the eapside, or Tenonitis, may be due to constitutional poison 

 or to infection following operations involving it, as in tenotomy of the ocular 

 muscles. It may be an extension from an inflammation of the eyeball. The inflam- 

 matory exudate in the capsule and adjacent tissues will sometimes cause a slight 

 exophthalmos, and the eye will be immobile. All the extrinsic muscles of the eye 

 pierce the capsule about the equator of the globe to reach their insertions in it. 

 Each muscle receives a tubular investment from the capsule, which fuses with the 

 proper sheath of the muscle and leaves a small bursa on the anterior surface of 

 each. To open the capsule for a tenotomy, the incision is made just back of the 

 cornea, and goes through only the conjunctiva and outer laver of the capsule. The 

 desired tendon is easily found and brought out with a hook, when it is divided. The 

 capsular prolongation about the tendon prevents retraction of the stump after the 

 division, and so preserves the function of the muscle. This is aided by expansions 

 of the capsule passing to the margins of the orbit and continuous with the perios- 

 teum. Those passing from the internal and external recti are stronger than the 

 others and are called the internal and external check ligaments. They are united 

 by a layer of fascia (suspensory ligament of the eyeball) passing under the eyeball 

 so that the eye is supported after the bony floor of the orbit has been removed, as 

 after excision of the superior maxillary bone. If the outer layer of the globe is left 

 after enucleation of the eye, the muscles will still have an attachment and be capable 

 of moving an artificial eye fitted to the stump. 



While the movements of the eyeball are free in all directions, as in a ball and 

 socket joint, no change in position of the eyeball, as a whole, takes place, as the 

 centre of rotation is about in the centre of the globe. By these movements the 

 image of the object to be especially seen is fixed upon the most sensitive part of the 

 retina. 



The internal rectus draws the ball directly inward and the external rectus 

 directly outward. The other four muscles, the superior and inferior recti and the 

 two oblique, have a complicated action. The upward and downward movements 



