The Book Never Written by Dr. Seuss’ Arch Enemy

Dr. Seuss’s arch enemy, Dr. Samiam, did not think much of the children’s book author. Dr. Samiam was extremely jealous of the fame Dr. Seuss enjoyed. In fact, Dr. Samiam thought he was the far better rhymer and he set out to prove it by writing a rhyming medical text on the thorax. He began to write it not here, not there, but in a house, and stored the manuscript in a box. He first set sight, with all his might, but try and try he could not rhyme mediastinum, he was certainly facing a plight.  Can you help Dr. Samiam find the posterior border of the anterior mediastinum in the images?

Identify the posterior border of the anterior mediastinum?