THE VERTEBRAL COLUMN. 481 



and pain occur from involvement of the spinal nerves. The tenth dorsal nerve 

 arrives anteriorly at the level of the umbilicus, the twelfth is midway between the 

 umbilicus and symphysis and also sends an iliac cutaneous branch a couple of inches 

 behind the anterior superior spine to the skin of the buttock, and below and in front 

 of the great trochanter. The abdomen above the external inguinal ring is supplied 

 by the hypogastric branch of the iliohypogastric from the first lumbar. 



Psoas abscess is common. The psoas muscle arises from the lower border of 

 the body of the twelfth dorsal and the bodies of all the lumbar vertebrae. The'pre- 

 vertebral fascia covering the bodies of the vertebrae is continued downward over the 

 psoas muscle as its sheath. Therefore when pus forms in the bone it enters the 

 sheath and follows it downward under Poupart's ligament, usually just outside, but 

 sometimes, as it gets still lower down, to the inside of the femoral vessels (Fig. 482). 

 At other times it works its way backward and points in the angle between the erector 

 spinae muscle and the twelfth rib, or along the edge of the erector spinae lower down, 

 or a little farther out above the top of the middle of the crest of the ilium at Petit' s 

 triangle (see page 394)- It may also find an exit through the great sacrosciatic 

 notch and point on the posterior aspect of the thigh. Pus originating in the cervical 

 region produces retropharyngeal abscesses, which, if involving the second to the 

 fifth cervical vertebrae, may either point in the pharynx or work outward to the 

 posterior edge of the sternomastoid muscle (see page 156). 



INJURIES OF THE SPINAL COLUMN. 



The vertebrae may be dislocated and fractured. 



Dislocation is rarer than fracture ; it is most common in the cervical region. 

 The cervical vertebrae have their articular facets sloping downward and backward, 

 hence dislocation occurs when the upper vertebrae are pushed in front of the lower. 



Mastoid process 

 Transverse process 

 of atlas 



Second cervical 

 vertebra 



FIG. 483. Anterior view, showing relation FIG. 484. Lateral view, showing relation of transverse 



of transverse process of atlas to tip of mastoid process of atlas to tip of mastoid process, 



process. 



The cervical spine normally has but slight rotation, hence when luxated one articu- 

 lar surface is rotated or pushed over and in front of the one below, the opposite 

 articular surface acting as the axis and the distance between the two articulations as 

 the radius of the arch in which the luxated parts move. The elevation of the lux- 

 ated articular process over the one below is favored by the bending of the spine 

 above toward the opposite side. The head is rotated and inclined toward the un- 

 injured side. Bilateral luxation is rare without associated fracture. It is produced 

 by anterior flexion, and the head and neck are inclined forward while the lower 

 vertebra of the dislocated joints inclines backward, producing a kyphotic condition. 



Luxation affects most often the fourth, fifth, and sixth cervical vertebrae. The 

 atlas may be dislocated forward or backward by rupture of the transverse ligament, 

 fracture of the odontoid process, or by a slipping of the process under the ligament. 



Luxation of the atlas laterally is very rare, owing to there being normally a total 

 rotation of 60 degrees. 



