0.34 THE MECHANICAL FUNCTIONS. 



erect posture of man, but an intermediate or 

 semi-erect position. 



This view of the living habits of the quadru- 

 mana will afford the key to most of the peculia- 

 rities of structure they present to our observation. 

 The head, being no longer suspended at the end 

 of a horizontal, or recurved neck, is, in the usual 

 attitude of the animal, supported chiefly by the 

 cervical vertebrae. The greater developement of 

 the brain, and more especially of its posterior 

 lobes, creates a necessity for an extension of the 

 occipital 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 articulation 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 uni- 

 versally 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 

 powerful muscles, and is an organ of great flexi- 

 bility and strength. Monkeys possess a distinct 

 clavicle, a lengthened humerus and femur, a 

 radius and ulna moveable upon each other, and 



