This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
While caries of any part of the vertebrae may occur from injury, it is almost always the result of tuberculous disease in the bodies; the pedicles, laminae, and processes remain unaffected. As the bodies become destroyed the anterior portion of the spine collapses, and this causes a projection of the spines of the affected vertebrae posteriorly or kyphosis. This projection of one or more spinous processes is the surest indication of spinal caries or Pott's disease.
There is also rigidity of the affected region. This is recognized by the attitude assumed and by having the patient, if an adult, bend the back anteroposteriorly. Small children should be placed flat on a table, face down, and then gradually raised by the feet. If the spine is normal the child will readily bend in the lumbar and lumbodorsal regions. The movable regions, embracing the cervicodorsal and dor-solumbar vertebrae, are the sites most frequently affected. Distention of the abdomen 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.
Fig. 482. - Psoas abscess originating from spinal caries of the dorsolumbar vertebra and following the psoas muscle to the groin. (From a sketch by the author of a specimen in the Mutter Museum of the College of Physicians).
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 prevertebral 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 spinas muscle and the twelfth rib, or along the edge of the erector spinas 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 vertebras, may either point in the pharynx or work outward to the posterior edge of the sternomastoid muscle (see page 156).