XII 



MECHANICS OF KESPIKATION 



407 



an intrathoracic cavity communicating with the exterior, and 

 subject, like the lungs, to all the changes of intrathoracic pressure 

 (Luciaui). The method of automatic registration of variations of 

 iutrathoracic pressure by the oesophageal sound, which we intro- 

 duced into the experimental technique of physiology in 1877, is 

 based on this fact. 



In conclusion it must be noted that the negative intra- 

 thoracic pressure also affects the soft movable portions of the 

 walls of the thorax, more particularly the diaphragm, by which its 

 floor is separated from the ab- 

 dominal cavity and the inter- 

 costal spaces. The former, as 

 well as the latter, are during 

 the expiratory position of rest, 

 and in the dead body, curved or 

 bent towards the thorax, where 

 pressure is negative, while they 

 are subjected externally to at- 

 mospheric pressure. 



IV. Let us now consider the 

 changes in form and dimensions 

 exhibited by the thorax during 

 the alternate movements of ex- 

 pansion (inspiration) and con- 

 traction (expiration), which 

 compose the respiratory rhythm. 



During inspiration the whole 

 thoracic cavity dilates more or 

 less, in its several diameters, in 

 proportion to the intensity of 

 contraction and the number of 

 muscles which come into play. 



The dilatation of all the 

 horizontal diameters of the 

 thorax is the effect of the rais- 

 ing of the ribs, which, with the 

 vertebral column, with which 

 they articulate posteriorly, and the cartilaginous prolongations 

 and the sternum, to which they are united anteriorly, form the 

 skeleton or rigid system of the thorax. 



The ribs, to the number of twelve on each side, constitute a 

 series of long, slender, arched bones, which start from the dorsal 

 vertebrae to extend outwards and forwards. They slant obliquely 

 from above downwards, so that their points of posterior articulation 

 are a little above the anterior end, which is united with the 

 sternum by means of the cartilaginous prolongations, directly 

 (first seven ribs) and indirectly (eighth, ninth, tenth ribs). The 





FIG. 173. Riht half of thoracic skeleton. 

 (Spaltenholtz.) 



