The thoracolumbar fascia was studied by dissection in ten adult human cadavers. The posterior layer of this fascia was found to consist of two laminae. The superficial lamina is formed by the aponeurosis of latissimus dorsl. The deep lamina consists of bands of fibers passing caudolaterally from the midline. Both laminae form a retinaculum over the back muscles, and the deep lamina constitutes a series of accessory posterior ligaments that anchor the L2 to L5 spinous processes to the ilium and resist flexion of the lumbar spine. The function of these ligaments is enhanced by the contraction of the back muscles and the action of certain, restricted portions of the abdominal muscles.