Complex sentence: The internal surface, smooth, and concave, is directed downward and a little inward: on its posterior part there is a short costal groove between the ridge of the internal surface of the rib and the inferior border.
Split into simpler sentences: The internal surface, smooth, and concave, is directed downward and a little inward: on its posterior part there is a short costal groove. The costal groove is a groove between the ridge of the internal surface of the rib and the inferior border.