The left bundle then divides into a fan-like sheet of fibers which spreads over the left side of the interventricular septum, sending Purkinje fibers to the base of both the anterior and the posterior papillary muscles where they are in direct continuity with fibers of the ventricular musculature. The anterior and posterior portions of this fan-like sheet are referred to as the anterior and posterior fascicles of the left bundle branch. These fascicles may be individually affected and cause specific electrocardiographic changes referred to as "hemiblocks" or "fascicular blocks."