RESPIRATION: THE MECHANISM OF BREATHING 359 



the same time, the contracted abdominal muscles press the walls 

 of that cavity against the viscera within it, and pushing these up 

 forcibly against the diaphragm make it very convex towards the 

 chest, and so diminish the latter in its vertical diameter. In very 

 violent expiration many other muscles may co-operate, tending 

 to fix points on which those muscles which can directly diminish 

 the thoracic cavity, pull. In violent inspiration, also, many extra 

 muscles are called into play. The neck is held rigid to give the 

 scalenes a firm attachment; the shoulder-joint is held fixed and 

 muscles going from it to the chest-wall, and commonly serving 

 to move the arm, are then used to elevate the ribs; the head is 

 held firm on the vertebral column by the muscles going between 

 the two, and then other muscles, which pass from the collar-bone 

 and sternum to the skull, are used to pull up the former. The 

 muscles which are thus called into play in labored but not in 

 quiet breathing are called extraordinary muscles of respiration. 



The Respiratory Sounds. The entry and exit of air are accom- 

 panied by respiratory sounds or murmurs, which can be heard on 

 applying the ear to the chest-wall. The character of these sounds 

 is different and characteristic over the trachea, the larger bron- 

 chial tubes, and portions of lung from which large bronchial tubes 

 are absent. They are variously modified in pulmonary affections, 

 and hence the value of auscultation of the lungs in assisting the 

 physician to form a diagnosis. 



The Capacity of the Lungs. Since the chest cavity never even 

 approximately collapses, the lungs are never completely emptied 

 of air: the space they have to occupy is larger in inspiration than 

 during expiration, but is always considerable, so that after a 

 forced expiration they still contain a large amount of air which 

 can only be expelled from them by opening the pleural cavities; 

 then they collapse almost completely, retaining within them only a 

 small quantity of air imprisoned within the alveoli by the collapse 

 of the small bronchi. 



The capacity of the chest, and therefore of the lungs, varies 

 much in different individuals, but in a man of medium height 

 there remain in the lungs after the most violent possible expira- 

 tion, about 1,000 cub. cent, of air, called the residual air. Aft.-r 

 an ordinary expiration there will be in addition to this about 

 1,000 cub. cent, of supplemental air; the residual and supplemental 



