240 MUSCLES OF FORCED EXPIRATION. 



Le., the position of expiration and tend to untwist themselves ; at 

 the same time, the elasticity of the distended lungs draws upon the 

 thoracic walls and the diaphragm. Lastly, the tense and elastic 

 abdominal walls, which, in man chiefly, are stretched and pushed 

 forward, tend to return to their non- distended passive condition when 

 the abdominal viscera are relieved from the pressure of the contracted 

 diaphragm. fWhen the position of the body is reversed, the action 

 of the weight of the chest is removed, but in place of it, there is the 

 weight of the viscera, which press upon the diaphragm.) 



The abdominal muscles [obliquus internus and externus, trans- 

 versalis abdominis and levator ani] are always active during laboured 

 respiration. They act by diminishing the abdominal cavity, and they 

 press the abdominal contents upwards against the diaphragm. When 

 they act simultaneously, the abdominal cavity is diminished throughout 

 its whole extent. The Triangularis sterni depresses the sternal ends 

 of the united cartilages and bones, from the third to sixth ribs down- 

 wards ; and the Serratus posticus inferior depresses the four lowest 

 ribs, causing the others to follow. It is aided by the Quadratus 

 lumbomm, which depresses the last rib. According to Henle, the 

 serratus posticus inferior fixes the lower ribs for the action of the slips 

 of the diaphragm inserted into them, so that it acts during inspiration. 

 According to Landerer, the downward movement of the ribs in the 

 lower part of the thorax dilates the chest. 



In the erect position, when the vertebral column is fixed, deep inspiration and 

 expiration naturally alter the position of the centre of gravity, so that during 

 inspiration, owing to the protrusion of the thoracic and abdominal walls, the 

 centre of gravity lies somewhat more to the front. Hence, with each respiration 

 there is an involuntary balancing of the body. During very deep inspiration, the 

 accompanying straightening of the vertebral column and the throwing backwards 

 of the head compensate for the protrusion of the anterior walls of the trunk. 



114. Relative Dimensions of the Chest. 



It is important, from a physician's point of view, to know the dimensions of the 

 thorax, and also the variations it undergoes at different parts. The diameter of 

 the chest is ascertained by means of callipers ; the circumference with a flexible 

 centimetre or other measure. 



In strong men, the circumference of the upper part of the chest 

 (immediately under the arms) is 88 centimetres (34*3 inches), in 

 females 82 centimetres (32 inches); on the level of the ensiform process 

 82 centimetres (32 inches) and 78 centimetres ( 3 0'4 inches) respectively. 

 When the arms are placed horizontally, during the phase of moderate 

 expiration, the circumference immediately under the nipple and the 

 angles of the scapulae is equal to half the length of the body; in man 



