Anthropology - Comparative Anatomy of the Shoulder Region

Comparative Anatomy of the shoulder region:

Compare the shoulder joint of the articulated skeleton of the chimpanzee with

the articulated human skeletons. Taking into account the form of locomotion of

chimpanzees, consider how their shoulder joint differs from that of humans.

There are similarities, including the shape of the humeral head, the extension of the

acromion process over the humeral head. The differences are:

a) the proportions of the scapula, where in humans the supraspinous fossa is relatively

small compared with the relatively large infraspinous fossa. In the chimp there is no

so much disparity in size compared between the fossae. As the fossae are attachment

sites for different rotator cuff muscles, this reflects the chimps' greater need for

stabilizing the shoulder joint while climbing.

b) The angle of the glenoid fossa, which faces more laterally in humans and more

cranially (upwards) in apes. This reflects the greater use of the human upper limb in

a lowered position, where the chimp uses the limb in a fully abducted position during

climbing.

Given the similarities and differences in the bony structure of the human and

chimpanzee shoulder region, what might have occurred in human evolution to

result in the susceptibility of the human shoulder joint to dislocation?

The relative instability caused by the reduced musculature of the rotator cuff, and the

lateral rotation of the glenoid fossa, means that the upper limb is unstable in full

abduction, or alternatively unstable when subject to pressure from above (as in hitting

the ground or an opponent with full force when tackling in a rugby game)

Compare the articulated pelves of human, gorilla, chimp, and orang-utan on the

demonstration table.

Apart from the obvious size differences, what are the differences in the following

and why are they present?:

The orientation of the iliac blade

In which plane is the iliac blade of the great apes oriented?

Almost