RESPIRATION. 399 



the form of a flattened dome, the highest part being formed by the central 

 tendon. The tendon consists of three lobes which are partially separated by 

 depressions. The right lobe, or largest, is the highest portion and lies over 

 the liver ; the left lobe, which is the smallest, lies over the stomach and the 

 spleen ; while the central lobe is situated anteriorly, the upper surface blending 

 with the pericardium. The central tendon is a common point of insertion 

 of all the muscular fibres of the diaphragm. In the passive condition the 

 lower portions of the diaphragm are in apposition to the thoracic Malls. 

 During contraction the whole dome is drawn downward, while the parts of 

 the muscle in contact with the chest are pulled inward. According to Hult- 

 kranz, the cardiac part of the diaphragm descends from 5.5 to 11.5 millimeters 

 during quiet inspiration, and as much as 42 millimeters during deep inspira- 

 tion. Not only is the height of the arch lessened, but there is also a tendency, 

 owing to the points of attachment of the diaphragm, toward the pulling of the 

 lower ribs with their costal cartilages and the lower end of the sternum inward 

 and upward ; this traction, however, is counterbalanced by the pressure of the 

 abdominal viscera, the latter being forced downward and outward against the 

 thoracic and abdominal walls. If this counterbalancing pressure be removed 

 by freely opening the abdominal cavity, especially after removing the viscera, 

 the lower lateral portions of the thorax will be seen during each inspiration to 

 be drawn inward. It is during labored inspiration only that this movement 

 occurs in the intact individual. 



When the diaphragm ceases to contract, the elastic recoil of the distended 

 lungs is sufficient to draw the sunken dome upward into the passive position. 

 This upward movement of the diaphragm is aided by the positive intra- 

 abdominal pressure exerted by the elastic tension of the abdominal walls 

 through the medium of the abdominal viscera. In forced expiration the 

 contraction of the abdominal muscles (p. 407) adds additional force. 



The quadrati lumborum are believed to assist the diaphragm by fixing the 

 twelfth ribs, or even lowering and drawing them backward duringdeep inspira- 

 tion. Each of these muscles arises from the ilio-lumbar ligament and the iliac- 

 crest, and is inserted into the transverse processes of the first, second, third, and 

 fourth lumbar vertebrae and the lower border of one-half of the length of the 

 last rib. These muscles are regarded by some physiologists as expiratory agents. 



The serrati postici inferloj-es similarly assist the diaphragm by drawing the 

 lower four ribs backward, and in deep inspiration also downward. They not 

 only thus oppose the tendency of the diaphragm to pull the lower ribs 

 upward and forward, which would lessen its effectiveness in enlarging the 

 vertical diameter of the thorax, but they contribute to this enlargement by 

 their backward and downward traction upon the rib- and the attached por- 

 tions of the diaphragm. These muscles pass from the spines of the eleventh 

 ami twelfth dorsal and first two or three lumbar vertebrae and the supraspi- 

 nous ligament to the lower borders of the ninth, tenth, eleventh, and twelfth 

 ribs, beyond their angles. 



Simultaneously with the contraction of the diaphragm the thoracic walls 



