1040 ORGANS OF SPECIAL SENSE 



best developed of these bands, the external check ligament, passes forwards and out- 

 wards to the outer angle of the orbit, helping to support the lachrymal gland on its 

 way, and is inserted near the orbital edge immediately behind the external pal- 

 pebral ligament. The inner band, or internal check ligament, is larger than the 

 outer, but not so thick; it passes forwards and inwards to be inserted into. the upper 

 part of the lachrymal crest and just behind it. These two bands, external and 

 internal, come from the sheaths of the corresponding recti muscles. From the 

 sheath of the superior rectus come two thin bands, one from each border. The 

 inner joins the sheath of the tendon of the superior oblique; the outer goes to the 

 external angle of the orbit, assisting in the support of part of the lachrymal 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 inner angle of the orbit close behind its margin, about 

 midway between the internal 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 or Tenon's 

 capsule. This is a thin, transparent tissue, situated immediately beneath the 

 posterior lamina of the muscle-fascia. It follows the curve of the sclerotic from 

 the insertion of the recti to about 3 mm. from the optic nerve entrance, when it 

 leaves the eyeball and blends with the posterior lamina of the muscle-fascia; the 

 combined membrane may be traced backwards, 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 supraraginal lymph-space. 

 Tenon's capsule 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 sclerotic in the form of a 

 line running around the globe, just anterior to the insertions of the four recti muscles. 

 Between this line and the corneal border, the conjunctiva is separated from the 

 sclerotic by the subconjunctival tissue, strengthened by a fine expansion of the 

 muscle-fascia. 



The inner surface of the capsule is smooth, and is only connected with the sclerotic 

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

 capsule, known as the interfascial or Tenon's space, is a lymph cavity, and per- 

 mits free movements of the eyeball within the capsule. 



Relation of Tenon's Capsule to the Oblique Muscles. The capsule surrounds the 

 posterior third of the inferior oblique and its tendon, running along its ocular sur- 

 face till it meets the fascial band coming from the inferior rectus (see above), and 

 forming a serous bursa on the superficial surface of the oblique near its insertion. 

 The tendon of the superior oblique for about its last five millimetres is invested 

 solely by Tenon's capsule; 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 Tenon's capsule. 



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. Its course is somewhat S- sria P e d; 

 thus, on entering the orbit, it describes a curve, with its convexity down and out, 

 and then a second slighter curve, convex inwards. Finally, it runs straight for- 

 wards to the globe, which it enters to the inner side of its posterior pole. 



In its passage through the optic canal 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, known as the intervaginal space, is subdivided by a fine pro- 

 longation of the arachnoid (the arachnoidal sheath) into two parts, viz., an outer, 

 narrow, subdural, and an inner, wider, subarachnoid space, communicating 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 inwards, which form a frame- 



