X = The xiphoid process
The xiphoid process is the smallest and most inferior region of the sternum, or breastbone. At birth it is a thin, roughly triangular area of cartilage that slowly ossifies into a bone and fuses with the body of the sternum.
The word xiphoid comes from the Greek word for sword-shaped, which describes its thin and pointed shape. It is widest at its superior end where it is attached to the body of the sternum by a thin, slightly movable fibrous joint (syndesmosis). From its syndesmosis it gradually goes down into to a point.
The xiphoid process begins as a structure made of hyaline cartilage at birth and childhood, slowly ossifying into a bony part of the sternum. In fact the ossification of the xiphoid process is so slow that it often does not end until an individual reaches the age of 40.
The function of the xiphoid process is that it acts as one of several origins for the diaphragm muscle that forms the floor of the ribcage and performs the vital process of respiration. The xiphoid process also acts as an insertion for the rectus abdominis and transverse abdominis muscles that compress and flex the abdomen. During cardiopulmonary resuscitation (CPR), the xiphoid process may be used as a bony landmark to determine the location for administering chest compressions. It is extremely important that pressure is not exerted on the xiphoid process during chest compressions as this can cause the xiphoid process to separate from the sternum, possibly puncturing the diaphragm or liver.
Even though this bone starts out made of flexible cartilage, it is usually considered an immobile joint, which means that it can’t move or bend with the body. Its flexibility is important where growth and development are concerned though. In humans it is located at the level of the 9th thoracic vertebra and the T6 dermatome. Many medical experts believe that the softer beginnings of this bone allow the rib cage and sternum the ability to grow, expand and adjust without putting too much tension on each other.