THE RESPIRATION 



77 



a smooth membrane called the pleura. This membrane 

 turns back and lines the chest wall, so that when the chest 

 expands, the two sleek membranes glide over each other 

 with far less friction than would be the case if the lungs 

 and chest wall were touching (Exp. 2). 



The Respiratory Muscles. (Repeat Exps. 13, 14, 15.) 

 The chief breathing muscles are the diaphragm (see Figs. 

 73 and 74), the muscles forming 

 the abdominal walls (see Fig. 

 44), and two sets of short mus- 

 cles (an internal and an external 

 set), between the ribs. They 

 are called intercostals. (They 

 are the flesh eaten when eating 

 pork ribs.) The diaphragm, 

 which is shaped like a bowl 

 turned upside down, rounds up 

 under the base of the lungs 

 somewhat like a dome and sepa- 

 rates the chest from the ab- 

 domen. Its hollow side is 

 toward the abdomen and its 

 edges are attached to the lowest 

 ribs and the vertebra of the 

 loins. Inspiration is brought 

 about by the rising of the ribs 

 and the descent of the dia- 

 phragm. Expiration takes 

 place when the ribs descend, 

 the abdominal walls draw in, 

 and the transmitted pressure lifts the relaxed diaphragm. 



Inspiration. To cause inspiration the diaphragm con- 

 tracts, it flattens and descends, since its edges are attached 



FIG. 73. VERTICAL SECTION 

 OF TRUNK, showing dia- 

 phragm, cavities of thorax and 

 abdomen. 



