Length of Cervical Vertebral Spinous Processes
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The seven cervical vertebrae, as well as the thoracic and lumbar vertebrae, have dorsal projections known as spinous processes. In the cervical region these processes provide attachment for nuchal muscles that insert on the posterior cranial vault and that function to support and move the head. In pronograde (that is, with a body more parallel to the ground), quadrupeal apes, the cranium projects anterior of the vertebral column, and large nuchal muscles are required to counteract gravitational forces that act to rotate the face downwards. Accordingly, chimpanzees, gorillas and orangutans possess enlarged spinous processes, which are some two- to five-times larger relative to their vertebral bodies than are those of humans. In these apes the fifth or sixth cervical vertebra tends to have the longest and most robust spinous processes. The orthograde (upright) body posture and bipedal locomotion of humans results in a cranium that is balanced atop the vertebral column and does not require much in the way of counterbalancing tension from the nuchal muscles. Thus in humans the nuchal musculature is slight and the spinous processes of the cervical vertebrae are relatively short and gracile. In humans the seventh cervical vertebra tends to have the longest spinous processes (and is therefore often called the “vertebra prominens”).
Earliest axial fossils from the genus Australopithecus, , Journal of Human Evolution, 2019/07/01/, Volume 132, p.189 - 214, (2019)