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If one said: My hand is a nazirite, and similarly, if he said: My foot is a nazirite, he has not said anything of consequence. However, if he said: My head is a nazirite, or: My liver is a nazirite, he is a nazirite. This is the principle: If one accepted naziriteship by means of an entity upon which life depends, i.e., a limb or a body part that he cannot survive without, he is a nazirite. Conversely, if he mentioned part of the body that is not essential for life, he is not a nazirite. In this case, as he referred to his hair, which is certainly not a vital part of him, he should not be a nazirite.
The talmud has a specific understanding of what anatomical organ is “vital” or essential for life.
We explore notions of vital organs in antiquity.