RESPIRATION: THE MECHANISM OF BREATHING 395 



represented by the rib. When the whole muscular sheet is fixed 

 above and contracts, it is clear that its lower end will be raised 

 more than any intermediate point, since there is a greater length 

 of contracting muscle above it. The elevation of the ribs tends 



FIG. 120. Portions of four ribs of a dog with the muscles between them, a, a, 

 ventral ends of the ribs, joining at c the rib cartilages, b, which are fixed to carti- 

 laginous portions, d, of the sternum. A, external intercostal muscle, ceasing be- 

 tween the rib cartilages, where the internal intercostal, B, is seen. Between the 

 middle two ribs the external intercostal muscle has been dissected away, so as to 

 display the internal which was covered by it. 



to diminish the vertical diameter of the chest; this is more than 

 compensated for by the simultaneous descent of the diaphragm. 

 The Lateral Enlargement of the Chest is brought about by a 

 rotation of the middle ribs which, as they are raised, roll round a 

 little at their vertebral articulations and twist their cartilages. 

 Each rib is curved and, if the bones be examined in their natural 

 position in a skeleton, it will be seen that the most curved part 

 lies below the level of a straight line drawn from the vertebral to 

 the sternal attachment of the bone. By the rotation of the rib, 

 during inspiration, this curved part is raised and turned out, and 

 the chest widened. The mechanism can be understood by clasp- 

 ing the hands opposite the lower end of the sternum and a few 

 inches in front of it, with the elbows bent and pointing down- 

 wards. Each arm will then answer, in an exaggerated way, to a 



