On the front of the thorax, one of the most important vertical lines is the midsternal line, the middle line of the sternum. On the front of the thorax, one of the most important vertical lines is the midsternal line, the middle line of the sternum. It can be interpreted as a component of the median plane. This article incorporates text in the public domain from page 1309 of the 20th edition of Gray's Anatomy (1918)