RESPIRATION: THE MECHANISM OF BREATHING 393 



radiate, downwards and outwards, to all sides; and are fixed by 

 their inferior ends to the lower ribs, the breast-bone, and the ver- 

 tebral column. In expiration the lower lateral portions of the 

 diaphragm lie close against the chest-walls, no lung intervening 

 between them. In inspiration the muscular fibers, shortening, 

 flatten the dome and enlarge the thoracic cavity, room for the 



FIG. 117. The skeleton of the thorax, a, g, vertebral column; 6, first rib; e. 

 clavicle; e, seventh rib; i, glenoid fossa. 



viscera thus displaced being secured by stretching the abdominal 

 walls; at the same time its lateral portions are pulled away from 

 the chest-walls, leaving a space into which the lower ends of the 

 lungs expand. The contraction of the diaphragm thus increases 

 greatly the size of the thorax chamber by adding to its lowest and 

 widest part. 



The Dorsiventral Enlargement of the Thorax. The ribs on the 

 whole slope downwards 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 at- 

 tached to it are raised, and so the distance between the sternum 

 and the vertebral column is increased. That this must be so will 

 readily be seen on considering the diagram Fig. 119, where ab 

 represents the vertebral column, c and d two ribs, and st the ster- 



