A macroscopic study of the lumbar regions of 4 adult bodies and of 4 foetuses has been made by the authors to specify the inferior insertion of the deep muscles of the back. These observations have demonstrated that the longissimus dorsi, and the ilio-costalis muscles are separated by a sagittal aponeurosis. The aponeurosis is a ventral reflection of the aponeurotic fascia of the Erector Spinae.