3 Latissimus dorsi Trapezius: From occipital bone, nape ligament and spine as far as twelfth dorsal, to clavicle, acromion and ridge of shoulder blade. Action: Extends head, elevates shoulder and rotates shoulder blade.
The back presents numerous depressions and prominences. This is due not only to its bony structure, but to the crossing and recrossing of a number of thin layers of muscles. It should be borne in mind that the superficial or outside layers manifest themselves only when in action. For this reason, under all changes of position, the spine, the shoulder blade with its acromion process, and the crest of the ilium, must be regarded as the landmarks of this region.
The spine is composed of twenty-four vertebrae. It extends the full length of the back, and its course is marked by a furrow. The vertebrae are known as the cervical, dorsal and lumbar. The cervical vertebrae are seven in number, and the seventh is the most prominent in the whole of the spine. It is known as vertebra prominens. In the dorsal region the furrow is not so deep as below. Here there are twelve vertebrae. When the body is bent forward, the processes of the vertebrae in this section are plainly indicated.
The spinal furrow becomes deeper as it reaches the lumbar vertebrae, where it is marked by dimples and depressions. It widens out, too, in this part of the body, and as it passes over the surface of the sacrum to the coccyx it becomes flattened. The average length of the spine is about two feet three inches.
The outer corner of the shoulder girdle is the acromion process, which is the high outer extremity of a ridge rising from the shoulder blade. The shoulder blade or scapula (spade) is a flat plaque of bone fitting snugly against the cage of the thorax, having a long inner vertical edge, parallel to the spine; a sharp lower point; a long outer edge pointing to the armpit; and a short upper edge parallel with the slope of the shoulder. The ridge, or spine of the scapula, starts at the spinal edge, about a third of the way down, in a triangular thickening, and rises until it passes high over the outer upper corner, where the shoulder joint ties, then turns forward to join with the collarbone at the acromion. The prominent portions are this ridge and the spinal edge and the lower corner. The upper outer corner is thickened to form the socket for the head of the humerus, forming the shoulder joint proper.
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 vertebrae when the spine is erect, in the middle vertebras when it is half flexed, in the upper vertebrae 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 becomes prominent under the skin. It produces easily fifty per cent of the whole movement of the shoulder.
Wedging of the Cage into the Hips
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