Muscles of the Back.—The chief back muscles are the trapezius and the latissimus dorsi, which together cover in the back pretty thoroughly. The [trapezius] arises from the occipital bone, the ligamentum nuchæ, and the spinous processes of the seventh cervical and all the dorsal vertebræ and is inserted into the outer third of the clavicle or collar bone and the acromion process and spine of the scapula or shoulder blade. It is thus triangular in shape and covers in the neck and shoulders, serving to draw the head back and to the side. It overlaps the latissimus dorsi.
The [latissimus dorsi] has its origin by aponeurosis from the spinous processes of the six lower dorsal and all the lumbar and sacral vertebræ, from the crest of the ilium or hip bone, and from the three or four lower ribs, swings across the side, dwindling in size, and is inserted by a small tendon into the bicipital groove of the humerus or upper arm bone, thus covering in the part of the back not covered by the trapezius. It draws the arm down and back, raises the lower ribs, and draws the trunk forward, as in climbing. The flat muscles of the back and abdomen have a tendency to flatten out into aponeuroses, such as occurs in the origin of the latissimus dorsi.
The [levator scapulæ], from the transverse processes of the upper cervical vertebræ to the posterior border of the scapula, serves to raise the angle of the scapula, and the [rhomboideus major and minor], from the ligamentum nuchæ, the seventh cervical, and the upper dorsal vertebræ to the root of the spine of the scapula, draw the inferior angle back and up.
Fig. 35.—Muscles of the trunk from behind (left side, superficial; right side, deep): 1, Sternomastoid; 2, splenius; 3, trapezius; 4, latissimus dorsi; 5, infraspinatus; 6, teres minor; 7, teres major; 8, deltoid; 9, external oblique of abdomen; 10, gluteus medius; 11, gluteus maximus; 12, levator anguliscapulæ; 13, rhomboideus minor; 14, rhomboideus major; 15, part of longissimus dorsi; 16, tendons of insertion of iliocostalis; 17, supraspinatus; 18, infraspinatus; 19, teres minor; 20, teres major; 21, serratus magnus; 22, upper, and 22´, lower part of serratus posticus inferior; 23, internal oblique; 24, gluteus medius; 25, pyriformis and superior and inferior gemelli; 26, 26´, portions of obturator internus; 27, tendon of obturator internus; 28, quadratus femoris. (Dorland’s Dictionary.)
The blood supply in the cervical region and about the shoulders comes from branches of the subclavian artery, such as the suprascapular and the transversalis colli. Lower down the supply comes from the posterior branches of the intercostals, dorsal branches of the lumbar, and branches of the internal iliac.
The muscles of the back are supplied by the spinal nerves, the spinal accessory also going to the trapezius muscle.