OF THE BACK. 349 



muscles draw the head backward. The Rectus capitis posticus major, owing to 

 its obliquity, rotates the cranium, with the atlas, round the odontoid process, 

 turning the face to the same side. The Multifidus spinge acts successively upon the 

 different parts of the spine ; thus, the sacrum furnishes a fixed point from which 

 the fasciculi of this muscle act upon the lumbar region ; these then become the 

 fixed points for the fasciculi moving the dorsal region, and so on throughout the 

 entire length of the spine ; it is by the successive contraction and relaxation of 

 the separate fasciculi of this and other muscles that the spine preserves the erect 

 posture without the fatigue that would necessarily have been produced had this 

 position been maintained by the action of a single muscle. The Multifidus spinse, 

 besides preserving the erect position of the spine, serves to rotate it, so that the 

 front of the trunk is turned to the side opposite to that from which the muscle 

 acts, this muscle being assisted in its action by the Obliquus externus abdominis. 

 The Complexi draw the head directly backward : if one muscle acts, it draws 

 the head to one side, and rotates it so that the face is turned to the opposite 

 side. The Superior oblique draws the head backward, and, from the obliquity 

 in the direction of its fibres, will slightly rotate the cranium, turning the face to 

 the opposite side. The Obliquus capitis inferior rotates the atlas, and with it the 

 cranium, round the odontoid process, turning the face to the same side. The 

 Semispinales, when the muscles of the two sides act together, help to extend the 

 spine ; when the muscles of one side only act, they rotate the dorsal and cervical 

 parts of the spine, turning the body to the opposite side. The Supraspinales and 

 Interspinales by approximating the spinous processes help to extend the spine. 

 The Intertransversales approximate the transverse processes, and help to bend the 

 spine to one side. The Rotatores spinae assist the Multifidus spinae to rotate the 

 spine, so that the front of the trunk is turned to the side opposite to that from 

 which the muscle acts. 



Surface Forms. The surface forms produced by the muscles of the back are numerous and 

 difficult to analyze unless they are considered in systematic order. The most superficial layer, 

 consisting of large strata of muscular substance, influences to a certain extent the surface form, 

 and at the same time reveals the forms of the layers beneath. The Trapezius at the upper part 

 of the back, and in the neck, covers over and softens down the outline of the underlying muscles. 

 Its anterior border forms the posterior boundary of the posterior triangle of the neck. It forms 

 a slight undulating ridge which passes downward and forward from the occiput to the junction 

 of the middle and outer third of the clavicle. The tendinous ellipse formed by a part of the 

 origin of the two muscles at the back of the neck is always to be seen as an oval depression, 

 more marked when the muscle is in action. A slight dimple on the skin opposite the interval 

 between the spinous processes of the third and fourth dorsal vertebras marks the triangular 

 aponeurosis by which the inferior fibres are inserted into the root of the spine of the scapula. 

 From this point the inferior border of the muscle may be traced as an undulating ridge to the 

 spinous process of the twelfth dorsal vertebra. In like manner, the Latissimus dorsi softens 

 down and modulates the underlying structures at the lower part of the back and lower part of 

 the side of the chest. In this way it modulates the outline of the Erector spinse ; of the Serratus 

 posticus inferior, which is sometimes to be discerned through it, and is sometimes entirely 

 obscured by it ; of part of the Serratus magnus and Superior oblique, which it covers ; and of 

 the convex oblique ridges formed by the ribs with the intervening intercostal spaces. The 

 anterior border of the muscle is the only part which gives a distinct surface form. This border 

 may be traced, when the muscle is in action, as a rounded edge, starting from the crest of the 

 ilium, and passing obliquely forward and upward to the posterior border of the axilla, where it 

 combines with the Teres major in forming a thick rounded fold, the posterior boundary of the 

 axillary space. The muscles in the second layer influence to a very considerable extent the surface 

 form of the back of the neck and upper part of the trunk. The Levator anguli scapulce reveals 

 itself as a prominent divergent line, running downward and outward, from the transverse pro- 

 cesses of the upper cervical vertebrae to the angle of the scapula, covered over and toned down 

 by the overlying Trapezius. The Rhomboidei produce, when in action, ^ a vertical eminence 

 between the vertebral border of the scapula and the spinal furrow, varying in intensity according 

 to the condition of contraction or relaxation of the Trapezius muscle, by which they are for the 

 most part covered. The lowermost part of the Rhomboideus major is uncovered by the Trapezius, 

 and forms on the surface an oblique ridge running upward and inward from the inferior angle 

 of the scapula. Of the muscles of the third layer of the back, the Serratus posticus superior 

 does not in any way influence surface form. The Serratus posticus inferior, when in strong 

 action, may occasionally be revealed as an elevation beneath the Latissimus dorsi. The Splenii 

 by their divergence serve to broaden out the upper part of the back of the neck and produce a 



