Ritual Sacrifice in Babylonian Exorcism
” … In addition, however, to burning the images of demons or sorcerers or throwing them into the water, a large variety of other symbolical actions are introduced in the incantation series, all falling within the category of sympathetic magic. The image is bound, hands and feet, so as not to be able to move, its eyes are pierced or filled with spittle, its tongue pulled out or tied, its mouth covered, or poison dripped into it or stuffed with dust, its body slit open  and the like; and thus mutilated, it is thrown into water or fire or on a dust heap.
From such rites it is not a long step to the endeavor to transfer the demon from the victim to some substitute a lamb, a pig or a bird, which appears then to have been offered up as a vicarious sacrifice for the life of the victim. 
“The lamb as a substitute for a man,
The lamb he gives for his life.
The head of the lamb he gives for the head of the man,
The neck of the lamb he gives for the neck of the man,
The breast of the lamb he gives for the breast of the man.”
The underlying thought is that the demon passes out into the animal which is offered to the gods, to appease their anger against the human sufferer. We are justified in drawing this conclusion from the caution expressly given  not to eat the animal which is declared to he taboo:
“Take a white lamb of Tammuz, 
Place it near the sick man,
Tear out its insides.
Place in the hand of the man,
And pronounce the incantation of Eridu.
That lamb whose insides thou hast torn out,
Cover it up as forbidden food for that man,
Consign it to the flame or throw it into the street.
That man shut up in a room and pronounce the incantation of Eridu.”
The animal has become unclean through the demon that has been transferred to it ; therefore it is not to be eaten, and while it is offered to the gods as a means of diverting their anger from the man on whom it has been visited, it is not a sacrifice in the ordinary sense. The demon may be also transferred to a bird which is caught for the purpose, slaughtered and cut up, after which the blood together with its skin and some portions of the body is burned in the fire  to the accompaniment of an incantation.”
Morris Jastrow, The Civilization of Babylonia and Assyria, 1915, np.