Why does nodding of the head signify yes and shaking of the head no?

In the late 1800s, louis pasteur was searching for a vaccine for anthrax in livestock. one french veterinarian had a local reputation for being able to cure anthrax by applying oils and wrapping the animal in cloth to induce a fever. pasteur also knew that some animals got well on their own when left untreated. pasteur tested the effectiveness of the local veterinarian's methods by injecting four cattle with anthrax bacteria. he then directed the veterinarian to perform his procedures on two cattle. the other two cattle were left alone. one cow from each group died and the other one in each group survived. pasteur dismissed the veterinarian's procedure as ineffective, but he also considered this to be a poor test. why would he consider this to be a poor test