A man in his early 60s presents to the emergency department (ED) with a 1-day history of pleuritic central chest pressure. He has a history of a recent myocardial infarction (MI) that was treated with ... However, I have yet to understand the geometrical meaning described on the Gathman's lecture, espeically, " infinitesimally " in one direction.

Understanding the Context

Obviously, it is impossible that the double point eat out all affine line. However, I do not surely catch why the double points occupies only very small area of affine line.