CHAPTER IX. 

 THE PHYSIOLOGY OF THE MUSCULAR SYSTEM, 



THE physiology of contractile tissues was treated under General 

 Physiology. This chapter should follow that, and presupposes a 

 knowledge of the human skeleton and skeletal muscles. 



I. ANIMAL MECHANICS. 



Animal mechanics is the application of the laws of mechanics to- 

 animal motion. The bones are used as levers; the articular surfaces 

 of bones usually serve as fulcrums, while the power is exerted by 

 the muscles. In a vast majority of cases the bones represent levers 

 of the third class in which rapidity of motion is attained at the 

 expense of power. In other words, the arrangement of the bone- 

 muscle organs is such that a contraction of a muscle moderate in 

 extent and rate of motion* is manifested by a movement of the limb 

 which is much in excess, as to extent and rate, of the movement of 

 the power. 



In solving problems in animal mechanics the principal factors to 

 be considered are: (1) The relative length of the two lever arms; 

 (2) the relative size of the muscles involved in any movement ; (3) the 

 direction in which the power acts, and (4) the weight to be moved. 



a. Problems in Animal Mechanics. Two typical problems in 

 animal mechanics are the following: 



1. Determine, in a particular case, the tension exerted upon the 

 tendo Achillis in supporting the weight (60 kilograms) of the subject 

 upon the ball of the foot. 



2. How much tension was there on the biceps tendon in the 

 subject upon your dissecting table when he held a 10-kilo iron ball 

 in the most advantageous position ? This is a typical problem and 

 its solution will make the difficulties to be encountered apparent. It 

 will also show that nothing more than an approximate solution can 

 be attained without an extended and detailed study. 



Solution. The principal muscle involved in the required action 

 being the biceps, the most advantageous position is the one in which 

 that muscle exerts its power in a line perpendicular to the lever. 

 Placing the subject's arm as nearly as possible in that position, one 

 takes the following measurements: (1) The long arm of the lever; 



