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The latissimus dorsi (plural: latissimi dorsi) is the larger, flat, dorso-lateral muscle on the trunk, posterior to the arm, and partly covered by the trapezius on its median dorsal region. The latissimus dorsi is a triangular, flat muscle, which covers the lumbar region and the lower half of the thoracic region, and is gradually contracted into a narrow fasciculus at its insertion into the humerus. This makes up some of the armpit and stretches down near the gluteus. It arises by tendinous fibers from the spinous processes of the lower six thoracic vertebrae and from the posterior layer of the thoracolumbar fascia, by which it is attached to the spines of the lumbar and sacral vertebræ, to the supraspinal ligament, and to the posterior part of the crest of the ilium. It also arises by muscular fibers from the external lip of the crest of the ilium lateral to the margin of the erector spinae, and from the three or four lower ribs by fleshy digitations, which are interposed between similar processes of the obliquus abdominis externus. From this extensive origin the fibers pass in different directions, the upper ones horizontally, the middle obliquely upward, and the lower vertically upward, so as to converge and form a thick fasciculus, which crosses the inferior angle of the scapula, and usually receives a few fibers from it. The muscle curves around the lower border of the teres major, and is twisted upon itself, so that the superior fibers become at first posterior and then inferior, and the vertical fibers at first anterior and then superior. It ends in a quadrilateral tendon, about 7 cm long, which passes in front of the tendon of the teres major, and is inserted into the bottom of the intertubercular groove of the humerus; its insertion extends higher on the humerus than that of the tendon of the pectoralis major. The lower border of its tendon is united with that of the teres major, the surfaces of the two being separated near their insertions by a bursa; another bursa is sometimes interposed between the muscle and the inferior angle of the scapula. The tendon of the muscle gives off an expansion to the deep fascia of the arm. The latissimus dorsi is responsible for extension, adduction, transverse extension, and internal rotation of the shoulder joint. It also has a synergistic role in extension and lateral flexion of the lumbar spine.
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In human anatomy, the trapezius is a large superficial muscle which extends longitudinally from the occipital bone to the lower thoracic vertebrae, and laterally to the spine of the scapula (shoulder blade). The trapezius has three functional regions: The superior region (descending part) supports the weight of the upper limb. The intermediate region (transverse part) retracts the scapulae. The inferior region (ascending part) medially rotates and depresses the scapulae.
The two trapezius muscles together resemble a trapezium, or diamond-shaped quadrilateral; the four points of the diamond are: the head (at the occipital protuberance), the two shoulders, and the middle of the back (at the spinous process of the twelfth thoracic vertebra, T12). Because the fibers run in different directions, it has a variety of actions, including: - scapular elevation (shrugging up or lifting the shoulders)
- scapular retraction (drawing the shoulder blades toward the midline)
- scapular depression (pulling the shoulder blades down)
It arises from the external occipital protuberance and the medial third of the superior nuchal line of the occipital bone (both in the back of the head), from the ligamentum nuchae, the spinous process of the seventh cervical (both in the back of the neck), and the spinous processes of all the thoracic vertebrae, and from the corresponding portion of the supraspinal ligament (both in the upper back). From this origin: - The superior fibers proceed downward and laterally. They are inserted into the posterior border of the lateral third of the clavicle.
- The inferior fibers proceed upward and lateralward. They converge near the scapula, and end in an aponeurosis, which glides over the smooth triangular surface on the medial end of the spine, to be inserted into a tubercle at the apex of this smooth triangular surface.
- The middle fibers proceed horizontally. They are inserted into the medial margin of the acromion, and into the superior lip of the posterior border of the spine of the scapula.
At its occipital origin, the Trapezius is connected to the bone by a thin fibrous lamina, firmly adherent to the skin. At the middle it is connected to the spinous processes by a broad semi-elliptical aponeurosis, which reaches from the sixth cervical to the third thoracic vertebræ, and forms, with that of the opposite muscle, a tendinous ellipse. The rest of the muscle arises by numerous short tendinous fibers.
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The rhomboid major is a muscle on the back that connects the scapula with the vertebrae of the spinal column. The rhomboid major arises from the spinous processes of the thoracic vertebrae T2 to T5 as well as the supraspinous ligament. It inserts on the medial border of the scapula, from about the level of the scapular spine to the scapula's inferior angle. The rhomboid major is considered a superficial back muscle. It is deep to the trapezius, and is located directly inferior to the rhomboid minor. As the word rhomboid suggests, the rhomboid major is diamond-shaped. The major in its name indicates that it is the larger of the two rhomboids. The rhomboid major helps to hold the scapula (and thus the upper limb) onto the ribcage. It also acts to retract the scapula, pulling it towards the vertebral column, and downwardly rotates the scapula with respect to the glenohumeral joint. Both collectively with levator scapulae elevate the medial border of the scapula.
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