862 



ORGANS OF SPECIAL SENSE 



On reaching the eyeball, the dural sheath is joined l)v the arachnoid, and turns 

 away from the nerve to be continued into the outer two-thirds of the sclerotic. 

 Similarly the pial sheath also here leaves the nerve, its greater part running into 

 the inner third of the sclerotic, Avhile a few of its fibres join the choroid; the inter- 

 vaginal space consequently ends abruptly in the sclerotic 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. It 

 is formed by processes passing out from the central fibrous cord at its ti'mdnation, 

 and l>y processes passing inwards from the pial sheath, sclerotic, and choroid. It does 

 not pass straight across the nerve, but follows the curve of the surrounding sclerotic, 

 being therefore slightly convex Ijackwards. The nerve-trunk here quickly becomes 

 reduced to one-half its former diameter, the fibres losing their medullary sheath, 

 and being continued henceforward as mere axis cylinders. Apart from the conse- 

 quent loss of bulk, this histological change may be readily recognised macroscopi- 

 cally in a longitudinal section of the nerve, its aspect here changing from opaque 

 white to semi-translucent grey. The part of the nerve within the lamina cribrosa 

 has already been seen in our ophthalmoscopic examination of the living eye. 



The optic nerve is mainly nourished by fine vessels derived from those of the 

 pial sheath, which run into the substance of the nerve in the processes above men- 

 tioned. In front of the entrance of the central retinal artery this vessel aids to 

 some extent in the blood-supply of the axial part of the nerve. 



Fig. 489. — The Blood-vessels of the Left Orbit, viewed from above. 



Sujiraorbital artery 

 LACHRYMAL GLAND 



Superior rectus IS^ 



EYEBALL 



External rectus 



Lachrymal artery 



Superior rectus, cut 

 Inferior ophthalmic vein 

 Superior ophthalmic vein 



OPTIC NER VE 



Common ophthalmic vein 



Commencement of superior 

 ophthahiiic vein 



TT Reflected tendon of superior 



n'l, oblique 



Tt'' Ophthalmic artery 



Anterior ethmoidal artery 



Posterior ethmoidal artery 

 Ciliary arteries 



Levator palpebrae, cut 

 Ligament of Zinn 

 Ophthalmic artery 



OPTIC COMMISSURE 



Internal carotid artery 



The Blood-vessels a\d Nerves of the Orbit 



As these structures will be more particularly described in other sections of this 

 work, a very short general account will suffice here. 



Arteries. — The main ])lood-sui)ply is afforded by the ophthalmic artery, a 

 branch of the internal carotid, which gains the orbit through the optic canal, where 

 it lies beneath and to the temporal side of the nerve. On entering the orbit it 

 ascends, and passes obliquely over the optic nerve to the inner wall of the orl)it; 

 in this early part of its course it gives off most of its branches, which vary much 

 in their manner of origin and also in their course. The arteries of the orbit are 



i 



