2. MUSCLE OF THORAX AND UPPER LIMB

Muscle of thorax and upper limb
• The thoracic wall is made up of five muscles: the external intercostal muscles, internal intercostal muscles, innermost intercostal muscles, subcostalis, and transversus thoracis.
• These muscles are primarily responsible for changing the volume of the thoracic cavity during respiration.
• Other muscles that do not make up the thoracic wall, but attach to it include the pectoralis major and minor, subclavius, and serratus anterior (anteriorly) and the primary costarum and serratus posterior superior and inferior (posteriorly).
• The deepest layer of the thorax wall is made up of the innermost intercostal muscles.
• They originate from the medial aspect of the costal groove of the rib above and insert onto the internal aspect of the rib below.
• These muscles are lined internally by the endothoracic fascia, which appears just superficial to the parietal pleura of the lungs.
• The diaphragm is another muscle in the thorax that serves as the main muscle of inspiration.
• Other minor accessory muscles that attach to the thorax include the scalene muscles and the sternocleidomastoid muscle, both of which may also minimally aid in respiratory efforts.
• Eleven pairs of intercostal muscles are in each of the intercostal spaces, arranged from superficial to deep.
• The most superficial layer is the external intercostal muscle, which originates from the inferior aspect of the rib above and inserts onto the superior aspect of the rib below in an inferomedial direction.
• The internal intercostal muscle forms the intermediate layer.
• These muscles originate from the lateral aspect of the costal groove of the rib above and insert into the superior aspect of the rib below in a direction perpendicular to the external intercostal muscles.
• This arrangement allows them to depress the ribs and subsequently reduce the thoracic volume during forced expiration.
• Muscles of the posterior thorax, such as the levatores costarum and serratus posterior superior and inferior, may also aid in respiration.
• The levatores costarum originates from the transverse processes of C7 to T11 and inserts onto the rib below.
• The serratus posterior superior attaches to ribs 2 through 5 and elevates them during inspiration whereas the serratus posterior inferior attaches the vertebrae to ribs 8 through 12 and depresses them during forced expiration.
• Other muscles of the thorax are involved in upper limb movement, which include the pectoralis major and minor, subclavius, and serratus anterior muscles.
• It functions to flex, adduct, and medially rotate the arm at the glenohumeral joint.
• Its clavicular head causes flexion of the extended arm while its sternoclavicular head causes extension of the flexed arm.
• The pectoralis minor muscle originates from the anterior surfaces of ribs 3 to 5 and the deep fascia
• It functions mainly to depress the tip of the shoulder and protract the scapula.
• The subclavius muscle originates at the costochondral junction of the first rib and inserts at the subclavian groove of the clavicle.
• It functions to stabilize the clavicle.

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