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Internal intercostal muscles also known as intercostalis internus aid in forced expiration (quiet expiration is a passive process). They originate on ribs 2–12 and have their insertions on ribs 1–11.Their fibers pass anterior and superior from the upper margin of the rib and costal cartilage to the lower margin of the rib above. [2]
The internal intercostal muscles (intercostales interni) are a group of skeletal muscles located between the ribs.They are eleven in number on either side. They commence anteriorly at the sternum, in the intercostal spaces between the cartilages of the true ribs, and at the anterior extremities of the cartilages of the false ribs, and extend backward as far as the angles of the ribs, hence ...
Pectoral muscles (colloquially referred to as "pecs") are the muscles that connect the front of the human chest with the bones of the upper arm and shoulder. This region contains four muscles that provide movements to the upper limbs or ribs. Deep muscles of the chest, including pectoralis minor, serratus anterior, and subclavius (Gray 1918)
The rib cage or thoracic cage is an endoskeletal enclosure in the thorax of most vertebrates that comprises the ribs, vertebral column and sternum, which protect the vital organs of the thoracic cavity, such as the heart, lungs and great vessels and support the shoulder girdle to form the core part of the axial skeleton.
In reference to the muscles of the thoracic wall, the intercostal nerves and vessels run posterior to the internal intercostal muscles: therefore, they are generally covered on the inside by the parietal pleura, except when they are covered by the innermost intercostal muscles, innermost intercostal membrane, subcostal muscles or the transversus thoracis muscle.
There are three layers of intercostal muscles. The external intercostal muscles are most important in respiration. These have fibres that are angled obliquely downward and forward from rib to rib. [2] The contraction of these fibres raises each rib toward the rib above, with the overall effect of raising the rib cage, assisting in inhalation.
A cutout of the thoracic wall showing the three layers of intercostal muscle - from the left wall. The muscles extend from the tubercles of the ribs behind, to the cartilages of the ribs in front, where they end in thin membranes, the external intercostal membranes, which are continued forward to the sternum.
The trapezius evolved separately, but the other three muscles in this group evolved from the first eight or ten ribs and the transverse processes of the cervical vertebrae (homologous to the ribs). [7] Functional demands have resulted in the evolution of individual muscles from the basal unit formed by the serratus anterior.