THE RESPIRATORY SYSTEM 133 



function of respiration. This function has, however, 

 been modified very distinctly by the habit of tree-climbing, 

 and especially by its most important development, the 

 emancipation of the fore-limb. The story of the changes 

 in the method of respiration is a singularly complicated 

 one, since it is so inextricably interwoven with the changes 

 produced in other systems that its main thread is apt to 

 be lost in the complications which occur in every chapter. 

 The primitive air-breathing Vertebrates draw air into 

 their lungs by creating a suction within the spaces inside 

 their bodies, and this they do by drawing their ribs 

 upwards and outwards towards their fore-limbs. They 

 " heave " their chests forwards, as one would pull out 

 one end of a concertina, and so suck air into the lungs 

 within their body cavity. 



Inspiration in these animals (tailed Amphibians and 

 unspecialized Reptiles) is produced by muscles that pull 

 the ribs towards the fore-limbs; expiration by a reversal 

 of the process, and by muscles which squeeze the internal 

 cavity of the body and bo drive the air out again. 



In the most primitive of the Mammals a great change 

 has come in, for the internal cavity oT the body is sub- 

 divided into a headward chest cavity, and a tailward 

 abdominal cavity, and the lungs are separated from the 

 abdominal viscera by a muscular partition the dia- 

 phragm. When the muscular diaphragm acts, it com- 

 presses the abdominal cavity this is~"its primitive 

 function but it can also create a suction in the chest 

 by pulling its floor tailwards, or, to continue the simile, 

 by pullmg~out the other end of the concertina. The 

 diaphragm therefore becomes capable of assisting in 

 drawing air into the lungs in inspiration. 



There are, therefore, in the Mammals two possible 

 mechanisms of inspiration; first, the original air-breathing 

 vertebrate method of elevating the ribs to the shoulder 

 girdle, and second, the new method of lowering the 

 floor of the chest cavity. The one was named by Sir 



