MKCIIAXICS or DIArilRAGM ;j!»3 



(3) by their (lilTerciil l)l()()d .supply I lie lonucr directly from the 

 aorta and the latter Iroiii the intercostal and internal manniiary 

 arteries ; and (4) by their difierent nerve supply, the crural being 

 served by the posterior branch of the phrenic nerve and the costal 

 by the anterior branch. Moreover, the two portions act some- 

 what differently, and further, people may be classed as having 

 respiration of a crural or of a parietal type depending on whether 

 the crural or the costal portions of the diaphragm are employed 

 dio'ing quiet breathing. The majority of individuals employ 

 both parts of the muscle in varying degrees. 



II. Mechanics of diaphragm. The crural portion, when it 

 contracts, acts as power to a lever of the third class. That is, 

 the fixed point or fulcrum is the point of origin of the sheet of 

 muscle on the vertebral column. The resistance to be overcome 

 is mainly the pressure of the contents of the abdomen, the peri- 

 cardial fixture and the point of insertion of the vena cava and other 

 vessels. They may, on the whole, be considered as a weight 

 applied at the central tendon. The power is thus between weight 

 and fulcrum — giving speed at the expense of strength. The sterno- 

 costal part of the muscle connects the lower ribs with the central 

 dome and acts as a lever of the same class as the crura. In this 

 case, however, the fulcrum is movable and is moved outwards by 

 other muscles. This results in a forward as well as a downward 

 movement of the dome. 



On the whole, the final result of the contraction of the dia- 

 phragm is similar to the descent of a piston — increasing the 

 capacity of the thorax vertically. The average descent is equiva- 

 lent to a drop of about \ in. all over. For ease in calculation, 

 say that the distance through which the diaphragm moved in an 

 ordinary quiet respiration w^ere 10 mm. and that the mean area 

 of the piston were 250 sq. cm., then the volume of air sucked 

 in w^ould be 250 c.c. (complemental pleura). Now as the tidal 

 air in quiet breathing is under 400 c.c, it will be clear that the part 

 played by the diaphragm in ordinary respiration is of major 

 importance. 



Synergic Muscles. Acting along with the diaphragm there are 

 those muscles w^iich abduct the lower ribs, viz. : the quadratus 

 lumborum and the deep costal muscles. These are synergic — 

 contracting synchronously with the diaphragm, and preventing 

 the lower ribs from being pulled inwards. In children w'here the 

 musculature is poorly developed one sometimes observes a distinct 

 depression of the lower chest wall at every inspiration. 



The antagonistic muscles together with the viscera form the 

 resistance against which the diaphragm moves. These are the 



