THE RESPIRATORY MECHANISM. 391 



nal between each pair of ribs (Fig. 130 B}, and have an oppo- 

 site direction, their fibres running upwards and forwards. In 

 forced expiration the lower ribs are fixed or pulled down by 

 muscles running in the abdominal wall from the pelvis to 

 them and to the breast-bone. The internal intercostals, con- 

 tracting, pull down the upper ribs and the sternum, and so 

 diminish the thoracic cavity dorso-ventrally. At 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 at- 

 tachment; 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 accompanied 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 bronchial tubes, and portions of lung 

 from which large bronchial tubes are absent. They are vari- 

 ously 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 com- 

 pletely 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 con- 

 tain a large amount of air which can only be expelled from 

 them by opening the pleural cavities; then they entirely col- 

 lapse, just as the bag in Fig. 125 would if the bottle inclosing 



