Internal Intercostalis Muscle
The internal intercostal muscles are a set of skeletal muscles and are located between the ribs. There are eleven muscles on each side. Among the cartilages of the true ribs they begin anteriorly at the sternum in the intercostal spaces, as well as at the anterior extremities of the cartilages of the false ribs, and extend backward till the angles of the ribs; so they are continuous towards the vertebral column by thin aponeuroses called the posterior intercostal membranes.
Pectoralis major is the largest muscle of the pectoral region which is lean fan shaped and emerges by two heads – small clavicular head and large sternocostal head.
Pectoralis major is inserted by a U-shaped (bilaminar) tendon on to the sidewise lip of the bicipital groove. The former lamina of the tendon is formed by the clavicular fibres, while posterior lamina is developed by sternocostal fibres. The two laminae are constant with each other inferiorly.
Muscles of the thoracic wall contain those that fill and support the intercostal spaces, those that pass between the sternum and the ribs, and those that cross several ribs between costal attachments.
Shape of Thoracic Cage
The thoracic cage resembles a truncated cone with its narrow end above and broad end below. The narrow upper end is constant above with root of neck from which it’s partly separated on either side by the suprapleural membranes. The broad lower end is totally split from abdominal cavity by the diaphragm, but gives passage to structures like, aorta, esophagus, and inferior vena cava.