176 



RELATIVE DIMENSIONS OF THE CHEST. 



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. 

 (When 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 [obliqtius internus and externus, transversalis abdominis 

 and levator ani] are always active during laboured respiration. They act by diminish- 

 ing 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 downwards ; and the 

 serratus posticus inferior depresses the lowest four ribs, causing the others to 

 follow. It is aided by the quadratus lumborum, 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 throw- 

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



114, RELATIVE DIMENSIONS OF THE CHEST. 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 (imme- 

 diately under the arms) is 88 centi- 



Fig. 141. 

 Sibson's thoracometer. 



Fig. 140. 

 Cyrtometer curve. Left side of the chest re- 

 tracted in a girl aged twelve. 



metres (34*3 inches), in females 82 centimetres (32 inches); at the level of the ensi- 

 form process 82 centimetres (32 inches) and 78 centimetres (30*4 inches) respec- 

 tively. When the arms are placed horizontally, during 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 82, and during deep inspiration 89 centi- 

 metres. The circumference at the level of the ensiform cartilage is 6 centimetres 

 less. In old people, the circumference of the upper part of the chest is diminished, 



