Breadth of Corpus Sternii
The sternum (breast bone) is located in the anterior portion of the thorax and provides a point of attachment for the ribs (via costal cartilages), and serves as the only skeletal point of attachment of the upper limb (via the clavicles) with the thorax. The sternum is divided into three anatomical regions (from superior to inferior): the manubrium, the body (also known as the corpus sternii, gladiolus, or mesosternum), and the xiphoid. Both humans and apes have broad sterna relative to monkeys. However, there is considerable variation in sternal width both within and between species of hominoids. Variation exists among humans in the development of the mesosternum, which in turn produces variation in the width and shape of the adult bone. In some individuals the mesosternum develops from four, vertically-arrayed, segments (known as sternebrae: sing. sternebra) that fuse to form the adult corpus sternii. These individuals (ca. 22% of people) have relatively narrow mesosterna. In most individuals (60% of people), the inferior two sternebrae are doubled (pairs of right and left segments, for a total of six sternebrae). These individuals have adult mesosterna that are narrow superiorly but wide inferiorly. In a minority of people (ca. 18%), all of the sternebrae are paired (for a total of eight), and these individuals have relatively wide mesosterna. Similar variation in developmental patterns produces the variation seen between species within the hominoidea. Chimpanzees rarely have any paired sternebrae, and their sterna bodies tend to be narrow compared to those of humans. In gorillas and orangutans, on the other hand, all of the sternebrae are usually paired, and these species have mesosterna that are wide relative to humans. A functional explanation for this variation has not been proposed.
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