the physical makeup of a person
In the text it says, "She has the constitution of a goat. She'll outlive us all." The constitution the Keller is talking about is one of a person, not a piece of paper. When trying to determine the definition of a word, it helps to put the definition in the sentence. A goat would not have a system of law or a written document. A goat would have a physical makeup. We are not talking makeup we put on your face. We are talking about how you're built - how you are made.