18 THE MECHANISM OF LIFE 



whales), a wing (in birds or insects), a " swimmeret " or paddle- 1 

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

 nished with claws and cutting organs (as in the case of the fore- j 

 limbs of a cat, the large chela?, or claw limbs of a crab or lobster, I 

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

 structure and degrees of freedom of movement are relatively I 

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

 limb is a freely movable appendage of relatively complex struc- 

 ture. 



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

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

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

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

 connected together in various ways. The bone of the upper arm 1 

 (the humerus) articulates with the shoulder-blade by a ball- and- 1 

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

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

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

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

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

 can be bent on the upper, or extended into the same straight line 1 

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



The radius, however, has a peculiar twisting movement on the 1 

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

 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 andj 

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

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

 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- j 

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

 plane; but the 'articulation between the proximal phalanx and' 



