THE SENSORI-MOTOR SYSTEM 



19 



its corresponding metacarpal is, to some extent, an universal 

 joint. Thus the thumb can be bent in the same way as the other 

 digits, but it can also be rotated or swung round in the same 

 way that the whole arm can. The fi^st finger has the same kind 

 of motion, but less of it, so to speak, and so also with the others. 

 The hind- limb of a quadrupedal animal, or the leg of a man, is 

 built on the same plan, and there may be very little difference 

 between the two limbs (as in a chimpanzee, for instance) with 

 regard to the degrees of mobility of the parts. In man. however, 

 the freedom of movement of the hind-limb is restricted, first by 



Shoulder bla.de (fixed bone) 





FIG. 3. MOVEMENTS OF THE BONES OF THE FORE-LIMBS. 



the rigid attachment of the pelvis to the vertebral column, and 

 second by the rigidity of the bones of the ankle relatively to those 

 of the wrist. 



Such an analysis of the skeleton as we have just indicated (and 

 which can easily be made more precise by the reader himself) 

 shows that the degree of mobility of any part of the body of the 

 higher animal is determined in general by the configuration of the 

 skeleton, and particularly by the shapes of the joints. The latter 

 make it possible for one bone to move on another in one or more 

 ways; thus we have the hinge- joints between the skull and the 

 atlas, the peg- and- socket joints between the atlas and the axis, 



