ceeding six pairs of ribs are attached between the segments. The seventh segment (xiphoid process) is not associated with a pair of ribs. It terminates in a round plate of cartilage.
Exercise VI. Draw and label the sternum.
There are thirteen pairs of ribs. A rib contains a dorsal and a ventral segment. The anterior seven pairs are the true ribs, for they are attached to the sternum. The six posterior pairs are the false ribs. The eighth, ninth, and tenth pairs attach to the posterior border of the seventh, and by cartilage to the fifth sternebra. The three posterior ribs are floating ribs, lacking ventral segments. The ribs are bow-shaped in general, the angle, or region of greatest convexity, being dorsal. Most of the ribs are distinctly flattened anteroposteriorly. From the fifth to the tenth pairs, inclusive, the distal end of the rib is twisted, so that the broad surface is posterolaterad. Compare the shapes of the different ribs.
The fifth rib may be used to illustrate the articular surfaces. The capitulum costae (head) is a slight swelling at the proximal end. Its circular medial surface articulates with the demifacets of two adjacent vertebrae, as previously described. The tuberculum costae (tubercle) is a dorsal enlargement two or three millimeters lateral to the head. It articulates with the transverse process of the vertebra, and is separated from the head by the collum costae (neck) of the rib.
The thoracic cavity, surrounded on the sides by the ribs, dorsally and ventrally by the backbone and sternum respectively, resembles an ice cream cone with the apex cut off. The cranial thoracic aperture is in front, the larger caudal thoracic aperture behind. Both are circumscribed by ribs, sternum, and backbone. The thoracic cavity in-
