MECHANISM OF BESPIRATIOX. 195 



^shall contract the capacity of the chest, the pressure in the 

 interior will be increased, and air will be expelled, until 

 the pressures within and without the chest are again 

 equal. In both cases the air passes through the trachea 

 and larynx, whether in entering or leaving the lungs, 

 there being no other communication with the exterior, and 

 the lung, for the reason before mentioned, remains under 

 all the circumstances described, closely in contact with the 

 walls and floor of the chest. To speak of expansion of the 

 chest, is to speak also of expansion of the lung. 



"We have now to consider the means by which the chest- 

 cavity is alternately enlarged and contracted for the en- 

 trance 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 iront 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 



