THE EYE 217 



and lateral thirds, the porus opticus comes into view. It con- 

 sists of an oval, whitish area with an elevated circumference, 

 termed the papilla of the optic nerve^ and a depressed centre, 

 termed the excavatio papilla. The arteria centralis retince 

 appears about the centre of the porus opticus and breaks up, 

 in a fairly regular manner, to supply the retina. Large 

 temporal branches pass to the supero-lateral and infero-lateral 

 quadrants, while corresponding branches are distributed to the 

 nasal half of the field. Two smaller branches, the upper and 

 lower macular arteries, pass laterally to the region of the 



FIG. 83. The Normal Fundus, showing the Porus Opticus (Optic Disc 

 and the Retinal Blood-vessels. (From SYM'S Diseases of the Eye.} 



macula lutea. These arteries have corresponding veins, and 

 the latter stand out more clearly on ophthalmoscopic examina- 

 tion owing to their greater lumina and thinner walls. The 

 walls of the retinal veins consist only of a layer of endothelial 

 cells and they are therefore liable to rupture following injury, 

 giving rise to retinal haemorrhages. No well-marked anastom- 

 oses occur between the various branches of the central 

 artery, and they communicate with one another only through 

 the capillary plexuses. On this account, while thrombosis of 

 the main trunk causes complete blindness, thrombosis of any 

 of the larger branches give rise to an area of scotoma. 



