4 THORAX 



if the student will close his nose and mouth and then try to 

 " take a breath " he will find that the diaphragm and the 

 muscles which move the ribs are not competent to create a 

 vacuum in the thorax, and nothing but an abortive inspiratory 

 movement is possible. Under ordinary circumstances, however, 

 as the walls of the thorax move in such a manner as to 

 increase the size of the cavity, atmospheric pressure (15 Ibs. 

 to the square inch) forces air through the air passages into 

 the distensible lungs, enlarging them as the thoracic cavity 

 increases in size, so that they fully occupy the expanding space 

 and no vacuum is formed. 



In inspiration, therefore, the cavity of the thorax increases 

 in extent, and the increased area is occupied mainly by air 

 forced into the elastic lungs, but partly also by blood which 

 flows into the great veins of the thorax. In expiration, when 

 the contraction of the muscles which raise the sternum and 

 ribs ceases, those bones return to their original positions and 

 the antero-posterior and transverse extents of the thorax 

 decrease. Simultaneously, the contraction of the diaphragm 

 having ceased, the tonicity of the abdominal muscles forces the 

 abdominal viscera back to their original positions, so restoring 

 the convexity of the diaphragm and diminishing the vertical 

 extent of the thoracic cavity. The combined effect of the 

 descending sternum and ribs, the ascending abdominal viscera, 

 forced upwards by the abdominal muscles, and the elasticity 

 of the lungs, overcomes the atmospheric pressure and air is 

 forced out of the lungs, or in other words the " breath goes 

 out " that is, expiration takes place. 



The two parts of the respiratory movement, the thoracic 

 part, produced by the movements of the ribs and sternum, 

 and the abdominal part, due to the action of the ab- 

 dominal muscles, may take place together, or independently, 

 producing then, respectively, " thoracic respiration " and 

 "abdominal respiration." 



The student should study the movements carefully, both 

 on himself and on his friends. 



THORACIC WALL. 



Two days at least should be devoted to the dissection of 

 the thoracic wall. 



