SPINAL MUSCULATURE 411 



is retained for the sake of freedom of movement. In man the spinal column, with 

 the exception of the sacral region, may be readily extended (bent backward) and 

 flexed (bent forward), aUducted. (bent to the side) and rotated. Freedom of 

 movement is greatest in the cervical and lumbar regions and is restricted by the 

 thorax in the thoracic region. The cervical region allows considerable flexion, 

 extension and rotation, but a more limited abduction. In the thoracic region 

 rotation and abduction are freer than flexion and extension. The lumbar region 

 is that in which the chief flexion and extension of the trunk takes place, but abduc- 

 tion and rotation are limited, especially the latter. In the isolated articulated 

 spinal column freedom of movement of the various parts depends chiefly upon the 

 thickness and elasticity of the intervertebral discs, upon the conformation of the 

 ^rticular processes, and upon the elasticity or arrangement of the various liga- 

 m~ents uniting the vertebrae. In the living bodyifreed^Q^mLQf., movement is further 

 restricted by the musculature and skeletal apparatus at tached to the column. 

 There is much individual variation in the flexibility of the vertebral column. 



The various movements of the column are produced partly by muscles which act directly on 

 it and partly by muscles which act on it through the head, thorax or pelvis. Most of the muscles 

 which act on it directly belong to the intrinsic dorsal musculature; that is, to musculature which 

 is derived from the dorsal divisions of the myotomes and is innervated by the dorsal divisions 

 of the spinal nerves. This musculature extends from the sacrum to the skull and is closely 

 applied on each side of the mid-dorsal line of the body to the backs of the vertebrae and the back 

 of the thorax (fig. 381). Its chief function is to extend the spinal column and head, hence the 

 old term applied to the superficial portion of this musculature "erector spinse." During the 

 development of the body, muscles belonging to the ventro-lateral thoracic musculature and to 

 the upper extremity come to overUe in part the intrinsic dorsal musculature. The trapezius 

 and rhomboid muscles which cover it in the cervical and thoracic regions, and the latissimus 

 dorsi which covers it in the thoracic and lumbar regions belong to the shoulder girdle and arm 

 and have already been described, p. 360. The serratus posterior superior, which overlaps it 

 in the upper thoracic region, and the serratus posterior inferior, which overlaps it at the junction 

 of the thoracic and lumbar regions, are derived from the intercostal musculature which is de- 

 scribed below, p. 422 (fig. 380). All of these muscles are innervated by the ventro-lateral divi- 

 sions of the spinal nerves. The levatores costarum (fig. 380), which extend from the transverse 

 process of the thoracic vertebr£B to the ribs, and which, in spite of their name, act chiefly on 

 the spinal column, are derived from the external intercostal musculature and are innervated by 

 the intercostal nerves. 



Ventral to the spinal column and closely applied to it there are a few muscles, the chief 

 function of which is to flex the column. All are supplied by branches from the ventro-lateral 

 divisions of the spinal nerves. Of these the longissimus colli and longissimus capitis and scalene 

 muscles have been described in connection with the muscles of the neck, p. 353. In the thoracic 

 region there are no muscles of this type. In the lumbar region there are four muscles on each 

 side, the pillars of the diaphragm, fig. 391, the psoas minor, fig. 391, the psoas major, fig. 391, 

 and the quadratus lumborum, fig. 391. All of these muscles are flexors of the spine, except the 

 quadratus, which is an extensor. The psoas major muscle is also a flexor of the thigh. Even 

 more powerful flexors of the column than those above mentioned are some of those which work 

 indirectly upon it through the leverage offered by the skuU (sterno-cleido-mastoid described 

 above, p. 347), and the thorax (the ventro-lateral abdominal musculature). 



Abduction and rotation of the spine are produced by contraction of muscles on one side 

 while the corresponding muscles on the other side are relaxed. See Table, p. 502. 



In the present section we shall confine our attention to the intrinsic dorsal musculature, 

 leaving for consideration elsewhere the other musculature which acts on the vertebral column. 



The intrinsic dorsal musculature is attached to the sacrum, to the ilium, to the 

 spines, transverse, and articular processes and laminae of the lumbar, thoracic, and 

 cervical vertebrae, to the backs of the ribs and to the base of the skull. Two great 

 longitudinal subdivisions may be recognised, a lateral, supplied by lateral branches 

 of the posterior divisions of the spinal nerves, and a medial, supplied by medial 

 branches. The lateral portion is further divisible into a superficial division, con-l 

 sisting chiefly of systems of muscles extending laterally from the spines of the verte-| 

 1 brae upward toward the transverse processes of the vertebrae, the ribs, and thej 

 \ mastoid process of the skull ; and a deep division, consisting of niuscles which ex- 

 * tend between successive transverse processes. The medial portion like^vise con- 

 ^sts of two parts; a superficial medial composed of fasciculi extending from in- 

 ferior to superior spines, best developed in the dorsal region, and a deep portion 

 consisting mainly of muscle fasciculi which pass from the transverse processes up- 

 ward toward the spines of vertebrae situated more cranially. In the neck the more 

 superficial extend to the base of the skull. Between the base of the skull and the 

 first two vertebrae there are several specialised muscles. There is also frequently 

 present the rudimentary sacro-coccygeus posterior described on p. 448, which 

 represents an extension into the caudal region of the intrinsic dorsal musculature. 



