The important changes in the human skeleton due to the assumption of an erect posture and bipedal locomotion are as follows:
1.In man the skull is not suspended but well-balanced or pivoted on the first vertebra of the vertebral column, which is inserted at the base of the skull. The foramen magnum is placed interiorly at the basal aspect of the skull, while in monkeys and apes its position is more posterior.
With the easy balancing of the head on the trunk the uncial region became less rugged suggesting less powerful neck muscles. The face became vertical.
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2.In the monkey and apes the vertebral column presents only two curves-a dorsal and a sacral, while in man there are four curves: cervical curve-convex in front; thoracic curve-concave in front; lumber curve-convex in front and pelvic curve-concave in front.
This arrangement makes the vertebral column S-shaped. Of these, thoracic and pelvic curves appear during fetal life but the other tow curves, namely, cervical and lumber, develop gradually, as the child endeavors to sit upright and then walk erect. Apart from the curves, direction of the spines is also changed.
3.With the change in the axis of the body the shape of thorax is also changed. The thorax becomes barrel-shaped. The ribs become more curved due to lateral expansion of the chest. The transverse diameter of the thorax becomes greater than the doors-ventral diameter.
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4.The pelvic girdle, in keeping with its major function of transmitting body weight, also undergoes certain changes. Its transverse diameter increases and the iliac bones become fan- shaped.
The upper opening and the cavity spread out laterally giving the pelvis a funnel shape with the outlet directed downward. The sacrum becomes short and broad. Ilium becomes short and broad and the iliac crest becomes more curved. Acetabulum tends to be nearer to the pubic syphilis. The sacral joint is tilted making an angle of about 60° with the horizontal surface.
5.The femur is to support the pelvis from below. It does so by its head articulating with the acetabulum which is directed down wards and outwards. The femur develops certain characteristics; the axis of the long stout neck makes a large angle with the axis of the shaft; lineal aspire appears at the posterior surface of the shaft; political surface is also developed with a marked concavity; the medial condoyle is at a more distal level than the lateral and assumes a larger size.
6.The changes in the foot are remarkable. It is no longer a grasping organ but purely a supporting organ. With the change of function some characters are also changed. The important changes are:
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(a) The great toe becomes non-opposable and comes in a line with the other toes. It contributes to the formation of the transverse arch of the foot. The head of the first metatarsal is included in the common transverse ligament.
(b) The medial and lateral arches are formed.
(c) The axis of the foot now passes through in between the first and second toes.
(d) The calcaneum becomes larger. The other tarsal bones develop in a wedge-shaped manner so as to contribute to the formation of the arches of the foot.