330 



MANUAL OF PHYSIOLOGY. 



FIG. 151. 



thoracic ventilation, and, in injury of the spinal cord, owing to 

 its isolated nervous supply, it may be called upon to do so. 



During ordinary quiet breathing in the male it does the greater 

 part of the work. 



When not in action, a great part of the muscular sheets of the 

 diaphragm lies in direct contact with the inner surface of the 

 lower costal part of the thoracic wall, the rest is higher than the 

 central tendon that forms the floor of the pericardium and is 

 fixed in one position. During inspiration these lateral parts are 

 separated from the ribs and drawn below the level of the central 



tendon by the contraction of the mus- 

 cular fibres. The separation is aided 

 by the abduction of the floating ribs, 

 which is accomplished by the quadratus 

 lumborum and the deep dorsal muscles, 

 In order that the diaphragm may 

 act to the best advantage, it is neces- 

 sary that its attachments be fixed by 

 the other muscles ; for when the quad- 

 ratus lumborum, levatores, and other 

 fixing muscles are not acting, the lower 

 floating ribs are drawn in by the dia- 

 phragm, and the power of that muscle 

 is much diminished by the approxima- 

 tion of its attachments. This may be 

 seen in spinal injuries when the respira- 

 tion is carried on by the diaphragm 

 alone. In these cases a circular furrow 

 marks the line of attachment of the muscle to the lower ribs 

 and their cartilages, which are drawn inwards during each 

 inspiration, the breathing being of course purely abdominal in 

 type. 



The Quadratus Lumborum, which passes from the pelvis to the 

 last rib, has, besides the action in aid of the diaphragm just men- 

 tioned, the power of drawing down the lower outlet of the thorax, 

 in which it is helped by other abdominal and dorsal muscles. In 



Diagram showing inter- 

 val between the position of 

 the diaphragm in expiration 

 (e, e) and inspiration (i,i). 

 The increase in capacity is 

 shown by the white areas. 



