abdominal muscle sacrospinalis A muscle that extends the length of the back and neck, that arises from the iliac crest, the sacrum, and the lumbar and two lower thoracic vertebrae, and that splits in the upper lumbar region into three divisions of which the lateral is made up of the three iliocostalis muscles, the intermediate is made up of the three longissimus muscles, and the medial is made up of the three spinalis muscles. BTO:0001652 BrendaTissueOBO erector spinae erector spinalis