XII 



MECHANICS OF EESPIKATION 



409 



elevation. According to Ebner, this dilatation is not perceptible in 

 the two first intercostal spaces, owing to the low inclination of the 

 first ribs and the minimal rise of the upper end of the sternum. 



The inspiratory dilatation of the vertical diameter of the 

 thorax is not directly visible from without, as it is produced 

 by the descent of the diaphragm, but it may be estimated from 

 the rise in the upper end of 

 the abdominal wall, owing to 

 the displacement of the viscera 

 that occupy the diaphragmatic 

 concavity. 



The inspiratory muscular 

 contractions which displace the 

 bones of the thorax from the 

 position of equilibrium are op- 

 posed 'by various resistances, 

 due to the weight of the parts 

 to be lifted, the elasticity of the 

 costo-vertebral ligaments of the 

 costal cartilages and bones, and 

 lastly to the elastic resistance 

 of the lungs which produces 

 negative intrathoracic pressure, 

 and the elastic resistance of 

 the gases in the alimentary 

 canal which work against the 

 downward movement of the 

 diaphragm. It follows that when 

 the inspiratory and dilating 

 mechanisms of the thorax cease 

 to work, the bones of the thoracic 

 cavity return spontaneously to 

 the mean position of equilibrium, 

 either from gravity or from the 

 elastic reaction of the ligaments, 

 cartilages, ribs, and lungs, as 

 well as of the stomach and in- 

 testines. We shall, however, see that the movements of expiration, 

 or retraction of the thorax, are always aided by expiratory muscular 

 contractions, which tend not only to bring the bones of the 

 thoracic cavity back into the position of equilibrium, but to force 

 them beyond this position by giving the ribs a twist from above 

 downwards, till in the forced or dyspnoeic respiration they reach 

 the maximal constriction of the thorax and diminution of its 

 several diameters. 



V. Of the inspiratory muscles the diaphragm (Fig. 175) is of 

 the first importance, owing to its conspicuous action. By the 



FIG. 175. Lower half of thorax with four lumbar 

 vertebrae. (Luschka.) Diaphragm seen from 

 the front ; a, (ith dorsal vertebra ; It, 4th 

 lumbar vertebra ; c, ensiform process ; d, d', 

 aorta, which enters diaphragm by special 

 aperture ; e, oesophagus ; /, aperture in tendon 

 of diaphragm for passage of vena cava inferior ; 

 1, 2, 3, trilobate expansions of tendinous centre ; 

 4, 5, costal portions, right and left, of dia- 

 phragm muscle ; 6, 7, right and left crura of 

 diaphragm ; 8, 8, internal intercostal muscles, 

 which are absent near the vertebral column, 

 where it joins the external intercostals ; 9, 9, 

 10, 10, subcostal muscles of left side. 



