540 THE MECHANICAL FUNCTIONS. 



tread. The muscles which raise the heel, and 

 which compose the calf of the leg, are of great 

 size and strength, and derive a considerable in- 

 crease of power from the projection of the bone 

 of the heel, into which their united tendons are 

 inserted. In all these respects the human struc- 

 ture possesses decided advantages over that of 

 the monkey, with reference to the specific objects 

 of its formation. 



It is impossible to doubt that nature intended 

 man to assume the erect attitude, when we 

 advert to the mode in which the head is placed 

 on the spinal column. The enormous develope- 

 ment of the brain, and of the bones which invest 

 it, increases so considerably the weight of that 

 part of the head, which is situated behind its 

 articulation with the vertebrae of the neck, that 

 the balance of the whole is much more equal 

 than it is in the monkey, where the weight of 

 the fore part very greatly preponderates. The 

 muscles which bend the head back upon the 

 neck, and retain it in its natural position, are 

 therefore not required to be so powerful as they 

 must be in quadrupeds, especially in those which 

 graze, and in which the mouth and eyes must 

 frequently be directed downwards, for the pur- 

 pose of procuring food. In man this attitude 

 would, if continued, be extremely fatiguing, from 

 the weakness of those muscles, and the absence 

 of that strong ligament which sustains the weight 



