1074 



SPECIAL SENSE ORGANS 



eter, the fibres losing, their medullary sheath, and being continued henceforward as mere axis- 

 cylinders. Apart from the consequent loss of bulk, this histological change may be readily 

 recognised macroscopically 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 noted in the ophthalmoscopic examination of the living eye (p. 1055). 

 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 mentioned. 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. 817. — Transvekse Section through Optic Nerve, showing the Relations of its 

 Sheaths and Connective-tissue Framework. 



Dural sheath 

 Arachnoidal sheath 



Subarachnoid space 



Subdural space - i ^ , ' f T 



Central retinal artery 

 Central retinal vein 



Pial sheath 



Connective-tissue frame- 

 work, with meshes in 

 which the nerve-fibre 

 bundles lie 



Fig. 818. — Longitudinal Section through Termination of Optic Nerve. 



"Pit in optic papilla 



Chorioid -r^sj." 



Short posterior 

 ciliary artery 



Central retinal 



vessels 



Dural sheath 



Pial sheath 



Arachnoidal sheath 



Optic nerve with its 

 connective -tissue 

 framework 



Retina 



^ ^^ Pigment epithelium 



'^^^^^^^ Suprachorioidal 

 — _si»*.^-s^ space 



Lamina cribrosa 

 Sclera 



The Blood-vessels and Nerves of the Orbit 



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

 work, a very short general account will suffice here. 



Arteries.— Tlie main b](j()d-supi)ly is afforded by the ophthalmic artery, a branch of the 

 internal carotid, which gains the ()rl)it through the optic foramen, where it lies below and lateral 

 to the nerve. On entering the orbit it ascends, and passes obliquely over the optic nerve to 

 the medial 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 

 remarkal)le for their tortuous course, for their delicate walls, and for their loose attachment to 

 the Hurrounrling tissues. Tlie opliMialmic artery gives off special branches in the orbit to the 

 lacrimal gland, the muscles, the retina (through the optic nerve), and the eyeball, as well as 

 to the mcningcH, the ethmoi<lal cells, and tlie nasal mucous membrane. Twigs from all the 

 different branches go to supply the fat, fascia' and ordinary nerves of the orbit. Branches 

 which leave the orbit anteriorly ramify on the forehead and nose, and also go to the supply of 



