by Aaron G. Filler, MD, PhD
Figure 9-6 - Flexion and extension mechanism in the lumbar spine.
A - In quadrupedal mammals, when the animal is standing or walking quadrupedally, the weight of the body suspended between the arms and legs pulls the spine into extension. A dense heavy elastic “inter-transverse” ligament between the LTPs is pulled taught and resists extension, providing the usual principal support of the body when it is horizontal (parallel to the ground). Contact between the base of the styloid process and the tip of the facet joint (“zygapophysis”) provides a “stylo-zygoid” contact that can also resist extension of the spine. When contact occurs, the “instantaneous” center of rotation moves up to the point of stylo-zygoid contact.
B - Homo sapiens lumbar spine. The intertransverse ligament system and the stylo-zygoid contact system are both lost eliminating the standard limitation mechanisms against hyperextension. The relevant anatomy is similar in Morotopithecus
A - From: Filler, A. G. (2007a). Homeotic evolution in the Mammalia: Diversification of therian axial seriation and the morphogenetic basis of human origins. (forthcoming).
B - From: Owen, R. (1857). Osteological contributions to the natural history of the chimpanzees (Troglodytes) and orangs (Pithecus). No. V. Comparison of the lower jaw and vertebral column of the Troglodytes gorilla, Troglodytes niger, Pithecus satyrus, and different varieties of the human race. Transactions of the Zoological Society of London, (printed 1862) 4(4): 89-115