152 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



are hinge-jointed and bound about by ligaments. The capacity of the hands to 

 turn palms up and palms down and to twist a screw driver and turn a door- 

 knob is due to the position of the radius on the thumb side. When the hand 

 is held palm up, the radius and ulna are parallel; when it is turned palm down, 

 the radius is twisted across the ulna (Fig. 9.15). In many vertebrates, except- 

 ing the primates, the radius and ulna are permanently crossed as in the frog, 

 and the front foot cannot be rotated. When a cat is washing her face, her paw 

 makes beautiful curves but never turns palm up. 



Five metacarpals form the middle bridge between the wrist and the fingers 

 (Fig. 9.15). The five phalanges, thumb and fingers, play the chief role in the 

 remarkable activities of the hand. The thumb turned palmside to the fingers 

 has taken great part in the development of art and science, actually in the 

 whole of history. The power of the human hand is in its ability to do a large 

 number of things moderately well, to scratch and dig in the soil, to write 

 letters, and do hundreds of other things. A ground mole can scratch and dig 

 in the soil with its front feet doing it extraordinarily well, but it cannot do 

 anything else with them. 



The pelvic girdle supports the trunk and, with the femurs firmly attached 

 to it, takes the first impact of all the jolts of locomotion. It is a shallow bowl 

 and in man bears the weight of the abdominal organs to a degree that is 

 uniquely human. As in all other mammals, the pelvic ring of bones of the 

 human female is the birth passage of the young. 



Each side of the pelvic girdle is composed of three fused bones (ilium, 

 pubis, and ischium). Where they meet a deep cavity receives the head of the 

 femur in a ball-and-socket joint (Fig. 9.8), the hip joint, the most deeply set 

 and strongly bound with ligaments of any joint in the body. As the shoulder 

 girdle and arms are constructed for pliability, so the pelvic girdle and legs are 

 built for strength. The neck of each femur is an arch that thins with age and 

 becomes very easily broken. 



In the leg the distal end of the femur articulates with the tibia and fibula 

 at the knee, a critical joint which is protected by an extra bone, the kneecap 

 (patella) (Fig. 9.15). The tibia and fibula are comparable to the bones of 

 the forearm but are far more rigid. Their distal ends articulate with the ankle 

 bones (tarsals), one of which forms the heel. These bones are bound so 

 tightly by ligaments that they are not allowed much movement; on the inner 

 side of the foot they are lifted up, and with the metatarsal bones take part in 

 forming the arch or instep. Actually this is a double arch, one across the foot 

 and the other running the length of it. The common flatfooted condition comes 

 about when the ligaments lose firmness and allow the tarsals to separate and 

 the metatarsals to drop down. Thus the foot loses its natural spring and lift. 



The activities of human toes are slight as compared with those of the 

 fingers. The first cause of their limitations is that the great toe cannot separate 



