18 THE MECHANISM OF LIFE 



whales), a wing (in birds or insects), a " swimmeret " or paddle- 

 like limb (as in many Crustacea), a mobile weapon or tool fur- 

 nisbed with. claws and cutting organs (as in the case of the fore- 

 limbs of a cat, the large chelse, or claw limbs of a crab or lobster, 

 or the arms and hand of a man), and so on. Sometimes its 

 structure and degrees of freedom of movement are relatively 

 simple (as in the side fin of the ordinary fish), but in general the 

 limb is a freely movable appendage of relatively complex struc- 

 ture. 



The reader may easily verify all that we are about to say by 

 looking at a human skeleton in a museum, and by observing the 

 modes of motion of the parts of his own body. The fore-limb, or 

 arm, then, contains a skeleton that consists of a number of bones 

 connected together in various ways. The bone of the upper arm 

 (the humerus) articulates with the shoulder-blade by a ball-and- 

 socket joint, so that it is freely movable in every direction, and 

 the shoulder-blade itself can be moved (as in " shrugging "), so 

 that the arm has thus additional freedom. The skeleton of the 

 forearm consists of two bones (the radius and ulna), and the latter 

 is articulated to the humerus by a hinge- joint, so that the forearm 

 can be bent on the upper, or extended into the same straight line 

 with the latter, giving 1 degree of freedom of movement. 



The radius, however, has a peculiar twisting movement on the 

 ulna, and as the wrist and hand are articulated with it they can 

 be turned round through a half- circle, the elbow- joint remaining 

 immovable. Thus the hand can be turned palm up (supinated) 

 or palm down (pronated). The hand itself is capable of 

 3 degrees of movement — that is, it can be turned in each of the 

 three directions of space. There are eight wrist bones arranged 

 in two rows, all articulating with each other, one row being 

 jointed with the radius and the other with the long bones of the 

 hand. Thus the hand, as a whole, can move up and down and 

 from side to side on the wrist, so that it has 3 degrees of 

 freedom. Each of its bones, the metacarpals, articulates with 

 the bone of a finger — that is to say, with the first, or proximal 

 phalanx, which articulates with the middle one, which finally 

 articulates with the terminal phalanx (the one that carries the 

 claw, or nail). The articulations between the terminal and 

 middle phalanges and the middle and proximal ones are hinge- 

 joints, so that the fingers can only bend on each other in one 

 plane; but the articulation between the proximal phalanx and 



