122 MAN AND THE LOWER ANIMALS n 



metatarsal is far less moveably articulated with 

 the tarsus than the metacarpal of the thumb with 

 the carpus. But a far more important distinction 

 lies in the fact that, instead of four more tarsal 

 bones there are only three ; and, that these three 

 are not arranged side by side, or in one row. One 

 of them, the os calcis or heel bone (ca), lies ex- 

 ternally, and sends back the large projecting heel ; 

 another, the astragalus (as), rests on this by one 

 face, and by another, forms, with the bones of the 

 leg, the ankle joint ; while a third face, directed 

 forwards, is separated from the three inner tarsal 

 bones of the row next the metatarsus by a bone 

 called the scaphoid (sc). 



Thus there is a fundamental difference in the 

 structure of the foot and the hand, observable 

 when the carpus and the tarsus are contrasted : 

 and there are differences of degree noticeable when 

 the proportions and the mobility of the meta- 

 carpals and metatarsals, with their respective 

 digits, are compared together. 



The same two classes of differences become 

 obvious when the muscles of the hand are com- 

 pared with those of the foot. 



Three principal sets of muscles, called '' flexors/' 

 bend the fingers and thumb, as in clenching the 

 fist, and three sets, the extensors extend them, 

 as in straightening the fingers. These muscles 

 are all "long muscles"; that is to say, the fleshy 

 part of each, lying in and being fixed to the bones 



