Ba was the Egyptian conception of the soul that was symbolized as a man-headed bird, which departed the body after death and winged its way in flight to the gods. It returned at intervals to the mummy for the purposes of comforting it and reassuring it concerning immortality. Occasionally it grasped the ankh and the nif and if frequently represented as flying down the tomb-shaft to the deceased, or perched on the breast of the mummy. It was sometimes carved on the mummy cases. In the Book of the Dead a chapter is devoted to the Ba, so the conception does not appear to have been purely spiritual. (see Ka) A.G.H.
Spence, Lewis, An Encyclopedia of Occultism, New York, Carol Publishing Group Edition, 1996, p. 57