Medially, it articulates with the manubrium of the sternum (breastbone) at the sternoclavicular joint. With their keeled sternum (breastbone), unlike the ratites, they are known as carinatae. An "aspirate" can also be obtained from the sternum (breastbone). Birds are the only vertebrates to have fused collarbones and a keeled breastbone. The shoulderblades are the scapular area and the breastbone is the sternal region. The shoulder blades are the scapular area and the breastbone is the sternal region. The central xiphoid process at the rear of the breastbone is V-shaped. The horses thorax is also deeper from the breastbone to the spine. The chest is deep and broad with a slightly protruding breastbone, with well-sprung ribs. The rear edge of the breastbone was the deepest point of the thorax. An alternate method involves an incision under the breastbone. Pain behind the breastbone or in the region around the stomach often feels like heartburn. Other characteristics include fused wrist bones similar to those of birds, and a keeled breastbone. The female skeleton also has wider and shorter breastbone and slimmer wrists. The most primitive known bird relative with a keeled breastbone is "Confuciusornis".