130 



IMOVEMEXT IN ANIMALS 



as we have at elbow and knee (Fig. 33). This is a 

 common type of joint not only in the higher animals, 



but also in all of the 

 groups of lower animals 

 that have jointed limbs. 

 Crayfish and lobsters 

 (Fig. 34), for instance, 

 have limbs with several 

 hinge joints in them, 

 these joints being set in 

 different planes so that 

 by contracting one or 

 the other the limb can 

 be bent now one way 

 and now another. In 

 our own bodies freedom 

 to move in more than 

 one direction is ob- 

 tained either by a ball- 

 and-socket joint as at 

 the shoulder and hip, 

 or by the kind of joint 

 we have at the wrist 

 where a number of 

 small bones can slide 

 over one another in va- 

 rious (hrections, and so 

 permit flexibility of 

 motion. Muscles are 

 arranged about the 

 joints in opposing pairs. 

 The number of such 

 pairs depends on the 

 ^ ^^ „ , variety of motions of 



lio. 3.S. — Human Skoloton, showing i- u u • • a • 



Rimplr hiriKr joint.s at ollnnv aiul knro; whlCh the JOmt IS Ca- 



ball and srukot jf.inKs at shoulder and pablc. What is meant 



hip; joint.s made up of .small lx)nes at , . . . 



wrist and ankle by OppOSmg pairS IS 



