A YEAK IN SCIENCE 



119 



movement of the ribs by the action of the muscles 

 between the ribs called the intercostals. 



To demonstrate this movement use the mechanical 



device. 



in 



The vertical bar AB represents the spinal column, 

 DF the sternum, CD and EF two ribs. Letter c repre- 

 sents the muscular attachment from the spine and 

 collar bone to the ribs. Letters a and b represent inter- 

 costal muscles. 



Note on the skeleton that the ribs slant downward 

 toward the sternum. On the device place the bar DF 

 lower than points CE. Measure the perpendicular dis- 

 tance between CE and DF. Then lift the ribs CD and 

 EF to a position perpendicular to AB (in the body this 

 movement is brought about by the contraction of the 

 intercostal muscles). Again measure the perpendicular 

 distance between CE and DF. How does it compare 

 with the first measurement? How has the position of 

 DF been affected? How does this movement affect the 

 size of the thoracic cavity? 



B. The thoracic cavity is enlarged vertically by the 



