54 



ANIMAL LOCOMOTION. 



from the shoulder by a short neck, as in the thigh-bone (femur). 

 Like the thigh-bone it is twisted upon itself and forms a screw. 

 The inferior extremity of the arm bone is furnished with 

 spiral articular surfaces resembling those found at the knee. 

 The spiral articular surfaces of the arm bone are adapted to 

 similar surfaces existing on the superior extremities of the 

 bones of the forearm, to wit, the radius and ulna. These 

 bones, like the bones of the leg, are spirally disposed with 

 reference to each other, and form a screw consisting of two 

 parts. The bones of the forearm are united to those of 

 the wrist (carpal) and hand (metacarpal and phalangeal) by 

 articular surfaces displaying a greater or less degree of 

 spirality. From this it follows that the superior extremities 

 of man greatly resemble his inferior ones ; a fact of consider- 

 able importance, as it accounts for the part taken by the 

 superior extremities in locomotion. In man the arms do not 

 touch the ground as in the brutes, but they do not on this 

 account cease to be useful as instruments of progression. If 

 a man walks with a stick in each hand the movements of his 

 extremities exactly resemble those of a quadruped. 



The bones of the human extremities (superior and inferior) 

 are seen to advantage in fig. 26; and I particularly direct 

 the attention of the reader to the ball-and-socket or universal 

 joints by which the arms are articulated to the shoulders 

 (x, x), and the legs to the pelvis (a, a), as a knowledge of 

 these is necessary to a comprehension of the oscillating or 

 pendulum movements of the limbs now to be described. The 

 screw configuration of the limbs is well depicted in the left 

 arm (x) of the present figure. Compare with the wing of the 

 bird, fig. 6, and with the anterior extremity of the elephant, 

 fig. 7, *p. 28. But for the ball-and-socket joints, and the 

 spiral nature of the bones and articular surfaces of the extre- 

 mities, the undulating, sinuous, and more or less continuous 

 movements observable in walking and running, and the 

 twisting, lashing, flail-like movements necessary to swimming 

 and flying, would be impossible. 



The leg in the human subject moves by three joints, viz., 

 the hip, knee, and ankle joints. When standing in the erect 

 position, the hip-joint only permits the limb to move forwards, 



