The extent to which neighboring muscles and the fascia contribute to the formation of the supraspinous and interspinous ligaments is not clear from the literature. The purpose of this investigation is to examine the midline attachments of tendons and the posterior layer of thoracolumbar fascia in order to determine their respective contributions to the formation of these ligaments throughout the thoracolumbar spine. Study of the dense connective tissue organization in the posterior ligamentous system was carried out on two cadavers serially sectioned into thin (2.5-mm) epoxy resin plastinated slices. Additional observations were taken from a gross anatomical study of the midline anatomy in two adult cadavers. The results show that the spinal attachments of trapezius, rhomboideus major and splenius cervicis combine with the deep fascia to form the supraspinous ligament in the upper thoracic spine. The posterior layer of the thoracolumbar fascia makes a major contribution to the supraspinous and interspinous ligaments in the lower thoracic spine. In addition to the posterior layer of thoracolumbar fascia, longissimus thoracis and multifidus combine to form the lumbar supraspinous and interspinous ligaments. Their spinal attachments produce a system of dense connective tissue with marked regional variation in fiber orientation and arrangement. The findings support the description of the supraspinous and interspinous ligaments as structures formed by both muscle tendons and aponeuroses along the length of the thoracic and lumbar spine, with regional differences in their connective tissue architecture.