SUPPORT AND MOVEMENT 



377 



skeleton must bear a greater portion of 

 the burden than other parts. In quadrupeds 

 the spinal column and legs resemble a sus- 

 pension bridge where the column functions 

 as tlie bridge itself and the legs as the sup- 

 porting piers at either end. In man and 

 other bipeds the body is elevated at one 

 end until it is in a vertical position which 

 requires more secure footings at the base. 

 This is essentially how a tall building such 

 as the Empire State Building in New York 



The axial skeleton 



The skeleton is usually divided into two 

 general parts: the axial region comprising 

 the skull, the column, and the ribs, and the 

 appendicular region which includes the ap- 

 pendages and their girdles ( Fig. 15-2 ) . All 

 of the bones are so securely tied together 

 with ligaments that they are torn apart 

 only under great strain. In spite of this 

 seemingly well-built frame, it is often badly 



Fig. 15-3. Comparative profiles of the dog, monkey, and man to show the relative shift in the 

 facial angle (see text). Because of the increasing size of man's brain, which has grown 

 over his shortened jaws, his facial angle has increased over that of lower forms. 



is constructed — merely a bridge stood on 

 end with elaborate footings. In the human 

 skeleton, this shift in weight has also neces- 

 sitated more rigid connection between the 

 supporting vertebral column, and the pelvic 

 girdle to which posterior supporting ap- 

 pendages are attached. This appears to 

 function in a fairly satisfactory manner, 

 although if one may judge by the number 

 of middle-aged people suffering from a 

 "sacroiliac" (the development of a faulty 

 union between the column and pelvic 

 girdle), it is clear that the arrangement is 

 not as good as it might be. 



mutilated in accidents as a result of our 

 modern means of transportation. 



The skull. The skull shows considerable 

 modification vv^hen compared to that of 

 lower vertebrates. The greater emphasis on 

 the cranial case compared to the mandible 

 is obvious when the heads of the dog, 

 monkey, and man, for example, are placed 

 side by side (Fig. 15-3). As the brain has 

 grown forward over the shortened jaws, it 

 is easy to see how the facial angle (angle 

 between a line drawn along the forehead 

 and one from the base of the nose to the 

 foramen magnum) has increased and how 



