860 ORGANS OF SPECIAL SEXSE 



To understand the special bands mentioned above, we must follow the sheath of 

 each rectus muscle forwards, when we find that, while it is rather loosely applied to 

 the muscular belly in its posterior two-thirds, it then suddenly liecomes thicker, 

 and is firmly attached to the muscle for some distance before finally leaving it, and 

 is thereafter often accompanied by some muscle-fibres. The best developed of 

 these bands, the external check ligament, passes forwards and outwards 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 pali)ebral 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, t)ne 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 lias a 

 special membrane enclosing its hinder two-thirds, usually called Tenon's capsule. 

 This is a thin, transparent tissue, situated immediately beneath 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 supravaginal 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 round the globe, 

 joining 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 Tenon's space, is a lymph cavity, and permits free move- 

 ments of the eyeball within the capsule. 



Relation of Tenon^ s Capsule to the Oblique Muscles. — The ca])sule 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 inferior rectus (see above), 

 and forming a serous bursa on the superficial surface of the oblique near its inser- 

 tion. 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, 

 extending from the optic foramen to the eyeball. Its course is somcAvhat S-shaped; 

 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 

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

 Besides curving as just described, the nerve also rotates on its long axis, so thal\ 

 the surface which is ]:)elow at the foramen becomes temporal Itefore entering the; 

 eyeball. 



In its passage through the optic canal the nerve is surrounded by a pro- 

 longation of the meninges. The dura mater splits at the optic foramen, partj 

 of it joining the periorbita, while the remainder continues to surround thej 



