Thoracic anatomy: a free MRCS Part A question

Along the midaxillary line the chest wall is covered by the fleshy digitations of serratus anterior, which arise from the outer surfaces of the upper eight or nine ribs, so a lateral drain passes through serratus anterior (or between its digitations) before reaching the intercostal muscles. Deep to serratus the track crosses the external intercostal, then the internal intercostal, then the innermost intercostal and endothoracic fascia before the parietal pleura. Pectoralis major lies anterior to the safe triangle and latissimus dorsi posterior to it, which is precisely why the triangle is defined between their borders; neither is traversed by a correctly placed drain. The internal intercostal is encountered only after the external layer. Reference: Gray's Anatomy for Students 4e.