THE OPTIC NERVE 1073 



corresponding recti muscles. From the sheath of the superior rectus come two thin bands, one 

 from each border. The medial joins the sheath of the tendon of the superior oblique; the 

 lateral goes to the lateral angle of the orbit, assisting in the support of part of the lacrimal 

 gland. The sheath of the inferior rectus is thickened in front, and, on leaving the muscle, goes 

 to the middle of the inferior oblique, splitting to enclose it; it then passes to be inserted into the 

 lower medial angle of the orbit close behind its margin, about midway between the medial check 

 ligament and the orbital attachment of the inferior oblique. 



3. In addition to its partial investment by the muscle-fascia, the eyeball has a 

 special membrane enclosing its hinder two-thirds, the fascia bulbi ("Tenon's 

 capsule"). 



This is a thin, transparent tissue, situated immediately internal to the posterior lamina of 

 the muscle-fascia. It follows the curve of the sclera from the insertion of the recti to about 3 

 mm. from the optic nerve entrance. There it leaves the ej-eball and blends with the posterior 

 lamina of the muscle-fascia; the combined membrane may be traced backward, enveloping the 

 optic nerve-sheath loosely, approaching it as it nears the optic foramen, but never actually 

 joining it. The interval between it and the nerve-sheath is called the supravaginal lymph-space. 

 The fascia bulbi first comes into relation with the muscles at the point where they are left by 

 their proper sheaths; it there invests their tendons, forms a small serous bursa on the anterior 

 surface of each, and adheres to the sclera along a line running around the globe, just anterior 

 to the insertions of the four recti muscles. Between this line and the corneal border, the con- 

 junctiva is separated from the sclera by the subconjunctival tissue, strengthened by a fine 

 expansion of the muscle-fascia. 



The inner surface of the fascia is smooth, and is only connected with the sclera 

 by a loose, wide-meshed areolar tissue. This interval between the sclera and 

 fascia, known as the interfascial (Tenon's) space, is a Ij-mph cavity, and permits 

 free movements of the eyeball within the capsule. 



Relation of the Fascia Bulbi to the Oblique Muscles. — The fascia surrounds the posterior third 

 of the inferior oblique and its tendon, running along its ocular surface till it meets the fascial 

 band coming from the inferioi- rectus (see above), and forming a serous bursa on the saperficial 

 surface of the oblique near its insertion. The tendon of the superior oblique for about its last 

 five millimetres is invested solely by the fascia bulbi; in front of this, as far as the trochlea, the 

 tendon lies in a membranous tube derived from the muscle fascia, the inner lining of which is 

 smooth, and may be considered as a prolongation of the fascia bulbi. 



The Optic Nerve 



The part of this nerve with which we have here to do lies within the orbit, ex- 

 tending from the optic foramen to the eyeball (fig. 813). The length of this 

 portion of the nerve is from 20 to 30 mm. and its diameter about 5 mm. Its course 

 is somewhat S-shaped; thus, on entering the orbit, it describes a curve, with its 

 convexity down and laterally, and then a second slighter curve, convex medially. 

 Finally, it runs straight forward to the globe, which it enters 3 to 4 mm. to the 

 medial side of its posterior pole. 



In its passage through the optic foramen the nerve is surrounded by a prolongation of the 

 meninges. The dura mater splits at the optic foramen, part of it joining the periorbita, while 

 the remainder continues to surround the nerve loosely as its outer or dural sheath. The nerve 

 is closely enveloped by a vascular covering derived from the pia mater, named accordingly the 

 pial sheath. The space between these two sheaths is subdivided by a fine prolongation of the 

 arachnoid (the arachnoidal sheath) into two parts, termed the intervaginal spaces [spatia inter- 

 vaginalia], viz., an outer, narrow, subdural, and an inner, wider, subarachnoid space, communi- 

 cating with the corresponding intracranial spaces. The arachnoidal sheath is connected with 

 the sheath on each side of it by numerous fine processes which bridge across the intervening 

 spaces. The pial sheath sends processes inward, which form a framework separating the bundles 

 of nerve-fibres; between the enclosed nerve-fibres and each mesh of this framework there is a 

 narrow interval occupied by tymph. The nerve-fibres are medullated, but have no primitive 

 sheath. About fifteen or twenty millimetres behind the globe the central vessels enter, piercing 

 obhquely the lower lateral quadrant of the nerve, and then run forward in its axis. They are 

 accompanied throughout by a special process of the pial sheath, which forms a fibrous cord in 

 the centre of the nerve. 



On reaching the eyeball, the dural sheath is joined by the arachnoid, and turns away from 

 the nerve to be continued into the outer two-thirds of the sclera. Similarly the pial sheath also 

 here leaves the nerve, its greater part running into the inner third of the sclera, while a few of its 

 fibres join the chorioid; the intervaginal spaces consequently end abruptly in the sclera around 

 the nerve-entrance. In this locality the connective-tissue framework of the nerve becomes 

 thicker and closer in its meshwork, and has been already alluded to as the lamina cribrosa 

 sclerae. It is formed by processes passing out from the central fibrous cord at its termination 

 and by processes passing inward from the pial sheath, sclera, and chorioid. It does not pass 

 straight across the nerve, but follows the curve of the surrounding sclera, being therefore slightly 

 convex backward. The nerve-trunk here quickly becomes reduced to one-half its former diam- 

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