Answer:
The paragraph in bold refers to Lincoln's belief that the constitution must be upheld and slavery should be prohibited, however it is not worth going to war over. In the analogy life is the constitution and the limb is the freedom of slaves. Sometimes freedom must be sacrificed for the wellbeing of the nation and to abide by the constitution, but fighting slavery is not worth destroying the constitution. Going to war would be like giving a life to save a limb; in the end it works but more is lost than what is gained.