MECHANICAL PHENOMENA OF RESPIRATION. 293 



Fig. 78. 

 Diagram of the intercostal muscles.* 



expiration from within outwards. We can also understand 

 how, in violent efforts of respiration these muscles contract, 

 not, however, in order to move 

 the ribs, but to support the 

 thoracic wall which their elas- 

 ticity alone would be powerless 

 to keep tense the spaces between 

 the bony arches. Hamberger's D 

 diagram, in this point of view, 

 shows contraction of the exter- 

 nal intercostal muscles during 

 inspiration, and of the internal 

 during expiration. 



The intercostal spaces are not 

 the only part of the thoracic 

 wall in which the muscular ele- 

 ments are so arranged as to 

 resist the changes of form in- 

 duced by variations in pressure : in forcible inspirations, 

 depressions, supra-sternal or supra-clavicular depression, 

 are produced at the summit of the thoracic cage, the base 

 of the neck. It is in these very parts that we find muscular 

 layers (subcutaneous) or muscular bands (omohyoid) leading 

 from the aponeuroses, and thus resisting the pressure from 

 without inwards, especially in yawning, sobbing, etc. 



We see, in short, that the transverse and antero-posterior 

 diameters of the chest are increased by the play of the costal 

 arches, set in motion by the contraction of a great number 

 of muscles, some of which are constantly in play, while others 

 are only accessory, and made use of in cases demanding 

 extraordinary power ; other muscles, the intercostal, in par- 

 ticular, serve only to keep the walls of the thorax in shape : 

 in normal respiration their elastic properties alone suffice to 

 produce this effect, but their contraction is necessary in 

 labored breathing. 



The enlargement of the vertical diameter is accomplished 

 by means of the diaphragm. This muscle forms the base 

 of the thoracic cone, and, as this descends, considerably 

 modifies the capacity of the cone : its action exactly resem- 



* Diagram known as Hamberger's. 



C C, D C7, Ribs raised. CD, DD', Ribs lowered. IT, Internal intercostal 

 muscles, extended when the ribs are raised (I), and relaxed when the ribs are 

 lowered (F). EE', External intercostal muscles, extended when the ribs are 

 lowered (E'), and relaxed when the ribs are raised (E). 



