What is "bell-magendie law"

In anatomy and neurophysiology, this is the finding that the anterior spinal nerve roots contain only motor fibers and posterior roots only sensory fibers and that nerve impulses are conducted in only one direction in each case. The nature and the physiology of the phenomena were described independently by the British anatomical scientist Sir Charles Bell and the French physiologist François Magendie, later confirmed by the German physiologist Johannes Peter Müller.