PRACTICAL CONSIDERATIONS: THE RETINA. 1469 



The integrity of the central artery of the retina is necessary to the preservation 

 of sight. The branches of this vessel are distributed to the retina only, and have 

 no communication with those of the other coats, nor do they anastomose with one 

 another. If the main artery or one of its branches is plugged with an embolus, the 

 area supplied by the blocked vessel is then deprived of sight. 



The retina may undergo inflammatory change in nephritis, syphilis, diabetes, 

 and other constitutional diseases. Of all these inflammations of the retina, that due 

 to kidney disease (albuminuric retinitis) is the most characteristic. Besides the 

 signs of general inflammation, as haziness of the retina, choked disc, distended 

 retinal arteries, or hemorrhages into the retina, pure white or even silvery patches 

 often occur ; they are due to fatty degeneration. Retinitis without these charac- 

 teristic changes may occur from albuminuria, so that the urine should be examined in 

 all cases of retinitis. 



The retina between the optic nerve and the ora serrata is held in apposition to 

 the choroid only by the support afforded by the vitreous body. It may be readily 

 detached from the choroid by such causes as injury, extravasation of blood or 

 serum between the two layers, or by tumors of the choroid. 



In contusions of the eye the retina is sometimes torn alone, although this is 

 rare. The retina does not tear as easily as the choroid, as is shown by the fact that 

 in ruptures of the choroid the retina is generally not lacerated. 



Glioma is the only tumor found in the retina, and occurs exclusively in children, 

 usually under three years of age. 



A rare tumor arising from the pars ciliaris retinae has been described, to which 

 the name terato-neuroma has been applied by Verhoeff. 



The Optic Nerve. The extraocular portion of the optic nerve has been de- 

 scribed elsewhere (page 1223). Likewise, the three sheaths the dural, the arachnoid 



FIG. 1226. 



Physiological excavation Lamina cribrosa 



Fibre-layer 



Visual cells \ 

 Choroid : 



Sclera 



Dural sheath - 



I'_L_-V Subarachnoid space 



Arachnoidal . f j 4 Subdural spa. 



sheath y 



Pial sheath i Y- '&'' 



r^^^..-. , Hl , JH 



Central retinal vessels within optic nerve 

 Section of eyeball through entrance of optic nerve. X 20. 



and the pial which, with the subdural and the subarachnoid lymph-spaces, are con- 

 tinued over the nerve as prolongations of the corresponding brain-membranes (page 

 949). On reaching the eyeball, the dural sheath bends directly outward, its fibres 

 commingling with those of the outer third of the sclera (Fig. 1226) ; the arachnoid 

 ends abruptly on the inner wall of the intervaginal space ; whilst the pia arches 

 outward to form part of the inner third of the sclera, but sends longitudinal fibres as 

 far as the choroid. As the nerve-fibres enter the eyeball, for convenience assuming 

 that they are passing from the brain toward the retina, they traverse a fenestrated 



