MECHANISM OF EXPIRATION. 363: 



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 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 mainly due- 

 to the diaphragm, which, when it contracts, adds to the 

 lowest and widest part of the conical chest cavity. Some 

 small widening is, however, brought about by a rotation of 

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

 a little at their vertebral articulations and twist their car- 

 tilages. 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 clasping the hands 

 opposite the lower end of the sternum and a few inches in 

 front of it, with the elbows bent and pointing downwards. 

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

 curved rib, and the clasped hands to the breast-bone. If 

 the hands be simply raised a few inches by movement at the- 

 shoulder-joints only, they will be separated farther from the- 

 front of the Body, and rib elevation and the consequent 

 dorso-ventral enlargement of the cavity surrounded will be- 

 represented. But if, simultaneously, the arms be rotated at 

 the shoulder-joints so as to raise the elbows and turn them 

 out a little, it will be seen that the space surrounded by the 

 two arms is considerably increased from side to side, as the 

 chest cavity is in inspiration by the similar elevation of the 

 most curved part or " angle" of the middle ribs. 



Expiration. To produce an inspiration requires con- 

 siderable muscular effort. The ribs and sternum have to 

 be raised; the elastic rib cartilages bent and somewhat 

 twisted; the abdominal viscera pushed down; and the ab- 

 dominal wall pushed out to make room for them. In ex- 

 piration, on the contrary, but little, if any, muscular effort 



