The rib cage is, in general, equipped with 12 pairs of matching bones, or 24 rib bones in all, while in some people this area of the anatomy may also have an additional bone, as is referred to as the cervical rib. When this alteration occurs, which in of itself is not strongly likely, most people will have just one cervical rib bone. In an even rarer instance, people may have a cervical rib bone. The cervical rib as a development in the human body is considered to be congenital, in the sense of becoming part of the person in question during the stage of fetal development. The cervical rib arises out of the seventh bone of the cervical vertebrae.
While the rib cage is a normal feature of the skeleton of vertebrate animals, the unusual nature of the cervical rib is unique to the human body. In some animals, a cervical rib appears normally, as is particularly common in the case of reptiles.
In that the presence of the cervical rib is not necessary or provided for in the anatomy of the human body, the cervical rib can consequently lead to adverse health effects when it appears. Specifically, the appearance of the cervical rib can lead to compression forces being exerted on the group of nerves referred to as the brachial plexus. People who are suffering from this problem due to having a cervical rib may thus lose a degree of functioning in their hand. A link between cervical rib bones and a heightened predisposition to developing cancer in childhood has been theorized but not proven.