244 THE HUMAN BODY. 



The Dorso-Ventral Enlargement of the Thorax. The ribs 

 on the whole slope downwards (Fig. 15) from the vertebral 

 column to the breast-bone, the slope being most marked in 

 the lower ones. During inspiration 

 the breast- bone and the sternal ends 

 of the ribs attached to it are raised 

 by muscles which pull on them, and 

 so the distance between the sternum 

 and the vertebral column is in- 

 creased. That this must be so will 

 readily be seen by examining the 

 diagram Fig 70, where ab represents 

 F IG . 70. Diagram iiius- the vertebral colunjn, c and d two 



trating the dorso -ventral in- ., .. . , 



crease in the diameter of the ribs, and st the sternum. The 

 thorax when the ribs are 



raised. continuous lines represent the natu- 



ral position of the ribs at rest in expiration, and the 

 dotted lines the position in inspiration. It is clear 

 that when their lower ends are raised so as to make 

 the bars lie in a more horizontal plane, the sternum is 

 pushed away from the spine, and so the extent of the 

 chest cavity between back-bone and breast-bone is in- 

 creased. 



Expiration. Inspiration requires a good deal of muscu- 

 lar effort. When the diaphragm contracts and flattens its 

 dome it has to push down the abdominal viscera on its 

 under side, and to press out the front wall of the abdomen 

 to make room for them. The ribs and breast-bone have 

 also to be pulled up out of their natural position of equilib^ 



What is the general direction of the ribs? How is it altered! 

 during inspiration? What is the consequence? Illustrate by refer 

 ence to a diagram. 



Why does inspiration require muscular effort? 



