RESPIRATORY MOVEMENTS 81 



intercartilaginous muscles, mk. In I the position in expiration is 

 represented; in II, that in inspiration. It is evident that in 

 II mr and mk are shorter than in I. 



In forced inspiration the following muscles aid in elevating the 

 ribs: scaleni, levatores costarum, serratus posticus superior, 

 sterno-cleido-mastoid ; also, after fixation of the arm, as by 

 gripping the table, the pectoralis major and minor and serratus 

 anticus major. During forced inspiration, the levatores alae nasi 

 contract, causing dilation of the nostrils, and also the crico-ary- 

 tenoideus postici which cause the dilation of the vocal bantls. 



The lowering of the ribs during expiration is brought 

 about by the sternum sinking by gravity and by the contrac- 

 tion of the internal intercostal muscles. These muscles 

 cross the externals and therefore act in the opposite direc- 

 tion. 



In forced expiration the ribs are lowered by the serratus posticus 

 inferior and latissimus dorsi ; further the upward movement of the 

 diaphragm is aided by the muscles of the abdominal walls and the 

 quadratus lumborum, which also aids in the lowering of the ribs. 



In the male, the respiration is chiefly effected by the lower 

 parts of the thorax, in the female by the upper. 



Normally, the intra-abdominal pressure sinks slightly 

 during inspiration and rises during expiration. Only when 

 the intestines are abnormally filled with food, faeces and 

 gases, does the intra-abdominal pressure rise during inspira- 

 tion and sink during expiration. 



The lungs are two sacs with extensible and elastic walls, 

 placed hermetically in the thoracic cavity, so that their 

 external surface (pleura pulmonalis) is everywhere in close 

 contact with the inner surface of the thoracic wall (pleura 

 costalis), but the two pleura are not grown together. The 

 inner surface of the lungs is much increased by thin mem- 

 branous projections which form the walls of the alveoli. 

 The internal space of the lungs communicates with the at- 

 mosphere by means of air-passages (bronchi, trachea, phar- 

 ynx, nose). The atmospheric pressure, therefore, presses 

 upon the inner surface of the lung and keeps the external 

 surface of the extensible lung-wall against the thoracic wall. 



