162 RESPIRATION. 



stances 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 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 

 bloodvessels, &c., and closed everywhere against the entrance 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 inspiration 

 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 intercostal which is be- 

 tween 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 con- 

 traction and consequent descent of the diaphragm the sides 

 of the muscle descending most, and the central tendon remain- 

 ing 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 roughly, 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 the antero-posterior diameter 

 of the chest by partially straightening out the angle between 

 the rib and its cartilage, and thus lengthening the distance 

 between its spinal and sternal attachments (Fig. 62, A). In 

 this way, at the same time that the ribs are raised, the sternum 

 is pushed forward. This forward movement of the sternum, 

 which is accompanied by a slight upward movement, is in part 



