206 RESPIRATION. 



cavity is alternately enlarged and contracted for the 

 entrance and expulsion of atmospheric air ; or, in technical 

 terms, for inspiration and expiration. 



Respiratory Movements. 



The chest is a cavity filled by the lungs, heart, and large 

 blood-vessels, etc., and closed everywhere against the en- 

 trance of air except by the way of the larynx and trachea. 

 It is bounded behind and at the sides by the spine and 

 ribs, and in front by the sternum and cartilages of the ribs. 

 Its floor is formed mainly by the diaphragm. 



The immediate inner lining of all these parts is the 

 outer or polished layer of the pleura ; and this membrane 

 also is stretched continuously across the top of the chest- 

 cavity, and mainly forms its roof. 



The enlargement of the capacity of the chest in inspira- 

 tion is a muscular act ; the muscles concerned in producing 

 the effect being chiefly the diaphragm and the external 

 intercostal muscles, with that part of the internal inter- 

 costal which is between the cartilages of the ribs. These 

 are assisted by the levatores costarum, the serratus posticus 

 superior, and some others. 



The vertical diameter of the chest is increased by the 

 contraction and consequent descent of the diaphragm, 

 the sides of the muscle descending most, and the 

 central tendon remaining comparatively unmoved; while 

 the intercostal, and other muscles just mentioned, by 

 acting at the same time, not only prevent the diaphragm 

 during its contraction from drawing in the sides of the 

 chest, but increase the diameter of the chest in the 

 lateral direction, by elevating the ribs ; that is to say, 

 by rotating them, to speak roughty, around an axis 

 passing through their sternal and spinal attachments, 

 somewhat after the fashion of raising the handle of a 

 bucket (fig. 62). This is not all, however. Another effect 

 of the contraction of the intercostal muscles is to increase 



