THE MECHANICS OF RESPIRATION 



335 



sixth to the tenth are joined to the sternum by the cartilages which unite 

 the sixth, seventh, eighth, ninth, and tenth, so that any movement in 

 which the ribs are raised is accompanied by an anterior movement of the 

 sternum (Fig. 112). The ribs are so articulated to the spinal column that 

 the inspiratory act causes the lateral and anterior part of each rib arch 

 to move forward and outward more than the one above it. 



In natural breathing in the standing or sitting posture there is a 

 slight extension of the spine during inspiration. This serves to increase 

 all diameters of the thorax and its absence is undoubtedly an important 



Fig. 112. Lower half of the thorax from the 6th dorsal to the 4th vertebra, seen from the 

 front, c, ensiform process; d, d' , aorta; e, esophagus; /, aperture in tendon of diaphragm for 

 passage of vena cava inferior; /, 2, 3, trilobate expansions of tendinous center of diaphragm; 4, 5, 

 costal portions, right and left, of diaphragm muscle; 6, right crus of diaphragm; 8, 9, internal 

 intercostal muscles, which are absent near the vertebral column, where it joins 9 and 9, the ex- 

 ternal intercostals; 10, 10, subcostal muscles of left side. (From L,uschka.) 



contributory factor in reducing the vital capacity of an individual when 

 lying on the back. Figures given by Hutchinson for the effect which 

 posture has on the vital capacity are of interest because of their bearing 

 on the cause of orthopnea. In the same individual he found the following 

 vital capacities: 



Standing 4300 c.c. 



Sitting 4200 c.c. 



Supine 3800 c.c. 



Prone 3620 c.c. 



