The human spinal column is made up of some thirty-three segments or vertebra : the length of the column of bones is much increased by intervertebral discs of fibro-cartilage placed between the segments.

The column has two main functions-(a) support of the trunk, etc., and transmission of its weight to the pelvis and lower extremities ; (b) protection of the spinal cord and its membranes. The structure of the whole column accords with the necessities of these functions, and thus the vertebra of which it is composed show their individual agreement with the general arrangement of the whole : they are modified in details according to their position, but they are all built on the same general principle.

This principle may be followed on a vertebra taken from the centre of the column and used as an illustration of the positions of the chief parts of a " typical " vertebra.