THE RESPIRATORY SYSTEM 135 



respiratory system becomes of increasing importance, 

 and the animal of necessity begins to depend more and 

 more upon its diaphragm as an inspiratory muscle 

 (see Fig. 50). 



We may say, therefore, that, as a general rule, pure 

 quadrupedal animals use their external respiratory system 



FIG. 49. DIAGRAM OF A QUADRUPEDAL ANIMAL, TO snow THE 

 MUSCLES PASSING FROM THE RELATIVELY FIXED FORE-LIMB 

 TO THE RELATIVELY MOVABLE RIBS. 



Only one muscle (serratus magnus) is represented. It produces 

 elevation of the ribs and the inspiration of air. 



most, but animals with emancipated fore-limbs depend 

 more and more upon their internal respiratory system. 

 So far is this the case, that in Man the original external 

 respiratory muscles are almost universally regarded as 

 no more than mere " extraordinary or accessory muscles 

 of respiration." 



It is perhaps worth turning aside to note how Man, 

 when he needs added respiratory mechanism, attempts 

 to take up a quadrupedal position, or at least change 

 the mobility of his fore-limb back again to stability, in 

 order to bring his primitive external respiratory muscles 

 into play. A runner who is " blown " will grasp his 



