CHAPTER XX 



THE RESPIRATORY SYSTEM 



WE will not deal directly with that portion of the re- 

 spiratory system which is concerned with the production 

 of voice, since the factors underlying the changes produced 

 in these structures are not those that bring about the 

 modifications in the disposition of the organs purety 

 concerned in breathing. 



Here we will only consider the effects produced upon 

 the chest and lungs, and the method by which air is 

 taken into the lungs. The alterations in the general 

 shape of the chest, which have differentiated the human 

 form from that seen in purely quadrupedal animals, are 

 very well known, and are discussed in all works dealing 

 with human morphology. 



These alterations are, as a rule, ascribed to the upright 

 posture and rightly; but, again, we must remember 

 that the upright posture need not mean the erect walking 

 posture, but may imply nothing more than mere arboreal 

 uprightness. In brief, we may say that a typical quad- 

 rupedal animal, such as a dog, is narrow-chested. Its 

 breastbone marks the keel of a chest, deep from breast- 

 bone to backbone, but narrow from side to side. An 

 upright animal, on the other hand, tends to have a broad 

 chest, the breastbone no longer protrudes as a keel, and 

 the chest is shallow from breastbone to backbone, and 

 broad from side to side. Man shows the extreme of this 

 flaT^eHng~^f~the~<jKesT; but the rounding of the quad- 

 rupedal chest is well seen in arboreal types quite low 

 down in the mammalian scale. Indeed, although we 

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