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.
A normal spine is either absolutely straight or very slightly convex to the right in the dorsal region, probably due to the increased use of the right hand. While scoliosis is called lateral curvature of the spine, it is not a simple lateral bending, but is a complex distortion (Fig. 481). R. W. Lovett has shown that a flexible straight rod can be bent in one plane either anteroposteriorly or laterally, but that a curved rod cannot be bent laterally without twisting or rotating Inasmuch as the human spine is curved convexly backward in the dorsal region and convexly forward in the lumbar region, lateral bending is accompanied by rotation of the vertebrae and their attached ribs. The bodies of the vertebrae are carried toward the side of the convexity of the curve and the ribs on that side project backward, producing a marked hump and often an elevation of the shoulder. As a primary curve forms, an attempt is made to restore equilibrium by bending the remaining portion of the spine in the opposite direction, hence the curves, if of long duration, are double or compound, and these secondary curves are called compensating curves. Marked lumbar curves are usually accompanied by prominence of the hip on the side of the convexity, but the pelvis usually remains level. Should the length of the limbs be unequal, allowing tilting of the pelvis, the prominence of the hip would be on the side of concavity. It is obvious that the weight of the body tends to aggravate these pathological curves. The treatment of scoliosis is directed to correcting these faulty curves by exercises and appliances intended to support and stretch the body on the contracted or depressed side and restore the tone and power to the relaxed muscles and tissues of the opposite side.
The spinal canal is formed by the laminae of the vertebrae arching over and uniting posteriorly. This union begins in the dorsal region and progresses towards the head and sacral regions. Failure of union constitutes spina bifida. It is most frequent in the lumbar and sacral regions. Usually a sac formed of the spinal membranes protrudes and contains the spinal cord flattened out like a strap passing down on its posterior surface, but sometimes the sac contains no nervous elements. Frequently the sac is so thin that it soon inflames, ruptures, and allows escape of the cerebrospinal fluid, and death ensues from meningitis. The parts below are not infrequently paralyzed and hydrocephalus may coexist. Operative procedures have been frequently successful in mild cases, but in extensive lesions they have been quite fatal, and even when primarily successful may be followed by the development of hydrocephalus.