The area occupied by the intrinsic postvertebral muscles varies in the different region of the back. This is associated with variations in the densities of muscle spindles in successive segments of the three columns of muscle. In the thoracic region, the lateral column has the highest density of spindles, whilst the intermediate and medial columns have steadily less. The attachment of iliocostalis and longissimus thoracis muscles to the ribs is such that contraction of these muscles can produce changes in the costotransverse joints. Conversely, movement in these joints might stimulate stretch receptors in the postvertebral muscles. There is collateral innervation of the costotransverse joints and the postvertebral muscles of the thoracic region by the posterior ramus. The finding of high densities of spindles in the thoracic region would therefore seem to support the hypothesis that these muscles probably monitor postural activity of the back as well as the rib cage.