The English word dog can be traced back to the Old English docga, a "powerful breed of canine". The term may derive from Proto-Germanic *dukkōn, represented in Old English finger-docce ("finger-muscle"). Due to the linguistically archaic structure of the word, the term dog may ultimately derive from the earliest layer of Proto-Indo-European vocabulary, reflecting the role of the dog as the earliest domesticated animal.

Different breeds of dogs have different eye shapes and dimensions, and they also have different retina configurations. Dogs with long noses have a "visual streak" which runs across the width of the retina and gives them a very wide field of excellent vision, while those with short noses have an "area centralis" — a central patch with up to three times the density of nerve endings as the visual streak — giving them detailed sight much more like a human's.

Dog Gates