Huginn and Muninn

The design

The Nordic Ravens, Huginn (represents the mind) and Muninn (represents the memory) in the world tree, Yggdrasil which has Algiz, the rune of the Valkyries and the Ravens carved into the trunk. 

In Norse mythology, Huginn (represents thought) and Muninn (represents memory) are a pair of ravens that fly all over the human world and bring information to the god Óðinn. 

When Óðinn was once asked if he didn't fear that one day, Huginn or Muninn wouldn't return from their daily flight, Óðinn responded by saying that he feared the loss of Huginn (thought) but was terrified of the loss of Muninn (memory).

Amongst the Norse, Ravens are generally considered to symbolise fylgja (generally a spiritual guide that follows you and warns you of dangers) and hamingja (happiness).

Yggdrasil is an immense mythical tree that connects the Nine Worlds in Norse Cosmology.