By some it is considered a reversion to the lower species, while others deny this and claim it to be simply a pathologic appendix.
Anomalies of the Spinal Canal and Contents .-- When there is a default in the spinal column, the vice of conformation is called spina bifida.
This is of two classes: first, a simple opening in the vertebral canal, and, second, a large cleft sufficient to allow the egress of spinal membranes and substance.

Figure 130 represents a large congenital sacral tumor.
Achard speaks of partial duplication of the central canal of the spinal cord.

De Cecco reports a singular case of duplication of the lumbar segment of the spinal cord.

Wagner speaks of duplication of a portion of the spinal cord.
Foot records a case of amyelia, or absence of the spinal cord, in a fetus with hernia cerebri and complete fissure of the spinal column.
Nicoll and Arnold describe an anencephalous fetus with absence of spinal marrow; and Smith also records the birth of an amyelitic fetus.
In some persons there are exaggerated curvatures of the spine.

The first of these curvatures is called kyphosis, in which the curvature is posterior; second, lordosis, in which the curvature is anterior; third, scoliosis, in which it is lateral, to the right or left.
Kyphosis is the most common of the deviations in man and is most often found in the dorsal region, although it may be in the lumbar region.
Congenital kyphosis is very rare in man, is generally seen in monsters, and when it does exist is usually accompanied by lordosis or spine bifida.