CHAPTER VI i. 

 IHtt FOUR METHODS OF RESPIRATION. 



In the consideration of the question of respiration, we 

 must begin by considering the mechanical arrangements 

 whereby the respiratory movements are effected. The 

 mechanics of respiration manifest through (1) the elastic 

 movements of the lungs, and (2) the activities of the sides 

 and bottom of the thoracic cavity in which the lungs are 

 contained. The thorax is that portion of the trunk between 

 the neck and the abdomen, the cavity of which (known as 

 the thoracic cavity) is occupied mainly by the lungs and 

 heart. It is bounded by the spinal column, the ribs with 

 their cartilages, the breastbone, and below by the dia- 

 phragm. It is generally spoken of as "the chest." It has 

 been compared to a completely shut, conical box, the small 

 end of which is turned upward, the back of the box being 

 formed by the spinal column, the front by the breastbone 

 and the sides by the ribs. 



The ribs are twenty-four in number, twelve on each 

 side, and emerge from each side of the spinal column. The 

 upper seven pair are known as "true ribs," being fastened 

 to the breastbone direct, while the lower five pairs are 

 called (false ribs) or "floating ribs," because they are not 

 so fastened, the upper two of them being fastened by carti- 

 lage to the other ribs, the remainder having no carti- 

 lages, their forward ends being free. 



The ribs are moved in respiration by two superficial 

 muscular layers, known as the intercostal muscles. The 

 diaphragm, the muscular partition before alluded to, separ- 

 ates the chest box from the abdominal cavity. 



In the act of inhalation the muscles expand the lungs 

 so that a vacuum is created and the air rushes in in accord- 

 ance with the well known law of physics. Everything 

 depends upon the muscles concerned In the process of res- 

 piration, which we may as, for convenience, term the 

 "respiratory muscles." Without the aid of these muscles 

 the lungs cannot expand, and upon the proper use and 



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