472 THE MECHANICAL FUNCTIONS. 



bone in that direction ; a portion of the weight to 

 be sustained by the atlas is accordingly thrown 

 behind the centre of motion, which is at its articu- 

 lation with the latter bone ; and this weight tends, 

 therefore, to balance that of the anterior part of 

 the head. Hence there is no need of the strong 

 cervical ligament, which is so universally met with 

 in quadrupeds ; and although this ligament exists 

 in the monkey, it is very slender, and of no very 

 great extent. 



Great mobility has been conferred on the spine 

 by the form of its articulations ; and the caudal 

 vertebrae are generally greatly multiplied to form 

 a tail of considerable length, which in the Ateles, 

 or spider monkey of America, is moved by power- 

 ful muscles, and is an organ of great flexibility 

 and strength. Monkeys possess a distinct clavicle, 

 a lengthened humerus and femur, a radius and 

 ulna moveable upon each other, and a hand nearly 

 approaching to the human construction. But the 

 thumb is less developed, and its muscles are much 

 weaker than in man. 



The bones of the pelvis, as well as those of the 

 leg, are elongated, for the purpose of giving 

 greater length to the muscles which are to move 

 their several parts; by this means, although the 

 force with which they act may be somewhat les- 

 sened, yet the velocity of the motion they produce 

 is increased in the same proportion. The fibula is 

 here a bone of more importance than in quadru- 

 peds ; for it performs a motion of rotation round the 

 tibia, analogous to that of the radius upon the ulna, 

 giving a great extent of action to the foot, and con- 

 verting the leg into an arm, as we have already 



