304 EXTERNAL RESPIRATION 



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 in the posterior wall of the thorax on the vertebral 

 column. The resistance to be overcome is mainly the pressure 

 of the contents of the abdomen, the pericardial 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 

 equivalent to a drop of about half an inch all over. For ease in 

 calculation, say that the distance through which the diaphragm 

 moved in an ordinary quiet respiration were 10 mm. and that 

 the mean area of the piston were 250 sq. cm., then the volume of 

 air sucked in would 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. 



Acting along with the diaphragm there are those muscles which 

 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 where 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 

 muscles of the abdominal wall, viz. : external oblique, internal 

 oblique, tranversalis and rectus abdominis on each side. 



(a) External Oblique (Descendens). This is the outermost and 

 largest of the paired abdominal muscles. It is muscular laterally 

 and tendinous in front. Above, it is attached to the lower eight 

 ribs by eight fleshy digitations from which the muscular fibres 

 pass obliquely downwards and forwards. The muscle fibres of 



