Table 2: Variation of the Central Retinal Vessels – Number and Location at the Optic Disc

	Main Retinal Artery	Main Retinal Vein
Mouse [5]	1 central	1 central	enters the ON at the level of the sclera
Rat [33-36	1 central	1 central	enters the ON at the level of the sclera
Rabbit [37,38]	1 central	4 peripheral
Quail, chicken [44-47]	avascular retina		vessels to the pecten
Pig [39]	6 lateral	1 central
Cat [41-43]	4-5 lateral	4-5 lateral
Dog [40]	4-5 lateral	1 central
Monkey [48]	1 central	1 central	enters the ON retrolaminar
Human [50,51]	1 central	1 central	enters the ON retrolaminar