"The muscular part of the chest wall is derived from myotome. In the abdominal region, the muscles arrange into three layers. The outer layer, Muscle external, internal oblique and transverse, is the same layer as the Muscle Pectoral major on the thoracic region. At the linea semilunaris, three oblique muscle layers are fused to form the rectus sheath. The sheath covers the longitudinal fiber of Rectus abdominis. This muscle originates from body and crest of the hipbone. Its fiber runs vertically and inserts at the anterior surface of the xiphoid process and 7th-5th costal cartilage. Similarly in cervical region, the longitudinal muscles are found by Sternothyriod Sternohyoid, Sternocleidomastoid, thyrohyoid, Geniohyoid, and omohyoid. In human, the longitudinal muscle of thoracic region is absent. Occasionally in some case, the longitudinal muscle of thorax has been found and called as rectus sternalis Superficial to the pectoralis major, some muscle fibres may pass vertically from the lower costal cartilages and rectus sheath to blend above with the sternocleidomastoid, (or) to be attached to the upper sternum (or) costal cartilages. These fibers may constitute the sternalis muscle (or) the rectus sternalis.
(Deepthi Simhadri, Suseelamma D and Praveen Kumar M- Uncommon Variation in Musculature of the Chest wall)."
Last date updated on August, 2020