Movement of flexion and extension occurs almost entirely in the waist or lumbar vertebrae. Movement of side-bending occurs throughout the whole length. Movement of rotation occurs in the lumbar verte-bne when the spine is erect, in the middle vertebrae when it is half flexed, in the upper vertebras when the spine is fully bent. In the lumbar vertebrae, the axis of this rotation is behind the spine; in the middle vertebrae it is neutral; in the upper dorsals it is in front of the spine.
Each vertebra moves a little, and the whole movement is the aggregate of the many little movements.
The shoulder blade slides against the surface of the cage of the thorax, in any direction, and may be lifted from it so that its point or its spinal edge become prominent under the skin. It produces easily fifty per cent, of the whole movement of the shoulder.
The profile of the sides presents a wide incomplete wedge, whose lines if prolonged would form an apex well below the buttocks. The surface proper of the back presents a great wedge, with base at the corners of the shoulders, with apex driven between the buttocks, buttressed on the sides by the lateral masses of waist muscle. With the addition of the neck, this becomes a diamond with a very blunt top.
From end to end vertically runs the dividing line of the spine; when bent, a series of knobs (tips of vertebral spines); when erect a groove except at the root of the neck, the spine of the seventh cervical vertebra. This serves as a sort of ridge pole for muscular tendons for neck and shoulders; and around it therefore is a flat unbroken fascia without muscular fibres, forming a lesser diamond nestling below the upper apex.
The Trunk. The Trunk, side view: 1 Latissimus dorsi. 2 External oblique.
From spine, sixth dorsal, to sacrum and iliac crest; passes inside of humerus to fasten to front side near head.
Draws arm backward and inward.
From eight lower ribs to iliac crest and ligament to pubis.
The Trunk. Muscles of the Trunk, back view.
3 Latissimus dorsi.
From occipital hone, nape ligament and spine as far as twelfth dorsal, to clavicle, acromion and ridge of shoulder blade.
Extends head, elevates shoulder and rotates shoulder blade.
Deltoid, page 104. Latissimus dorsi, page 162.
The trapezius is a diamond-shaped muscle; with upper apex at the base of the skull, lower apex well below the shoulder blades, and corners at the shoulder girdle opposite the deltoid, as though it were a continuation of that muscle.
From the sacrum the muscles diverge upward, while the lower ribs and lower corner of the shoulder blade diverge downward, making lesser diamonds of various definiteness of outline.
The ridge of the shoulder blade is always conspicuous, pointing diagonally toward the corner of the shoulder. It sets at a fixed angle with the spinal edge (more than a right angle) and at a right angle with the lower turned-out corner.
In relaxation, both ridge and blade are ridges under the skin, and are converted into grooves by the muscles bulging in contraction.
Of these muscles, those on either side of the ridge are easily recognizable - the deltoid, below and outside, and trapezius, above and inside, but the trapezius also spreads from the inner end of the ridge to well down the spine. Under this, helping to form the bulge, are the rhomboidei, extending from the blade diagonally upward to the spine, and the levator anguli scapulae, from its upper corner almost vertically to the top of the neck.
The Trunk. Trunk, back view. Masses and Their Movements: Tilting and Twisting.
The Trunk. The Cage of the Torse.
The Trunk. Wedging of the Cage into the Hips.
Three bones make the pelvis; two innominate (without a name) bones and one sacrum (sacrificial bone).
The sacrum is a wedge about the size of the hand but more perfectly shaped, bent like a half-bent hand, and carrying a very small tip about as big as the last joint of the thumb (coccyx). It forms the central piece in the back, curving first back and down and then down and in.
The two innominate bones are formed like two propellers, with triangular blades twisted in opposite directions. The rear corners of the top blades meet the sacrum in the back, and the front corners of the lower blades meet in front to form the symphysis pubis. The hip socket itself forms the central point for the shaft. The two blades stand at right angles to each other.
The upper blade is called the ilium, the lower is called the pubis in front and the ischium behind, with an opening between. The only superficial parts are the top of the upper blade (iliac crest) and the front tip of the lower (symphysis pubis).