486 DISSECTION OF THE ABDOMEN. 



When the diaphragm descends it changes its shape. The central tendon, 

 which moves but slightly, remains the highest part of the arch, whilst the 

 sides which contract freely are sloped from the tendon to the wall of the 

 thorax. During the ascent the midriff retains nearly the same form as in 

 :i state of rest, for the tendon is the lowest part of the arch, and the bulges 

 on the sides reach rather higher. 



AVith the movement of the diaphragm the size of the cavities of the 

 abdomen and thorax will be altered. In inspiration the thorax is enlarged, 

 and the abdomen diminished ; and the viscera in the upper part of the 

 latter cavity, viz., liver, stomach, and spleen, are partly moved from be- 

 neath the ribs. In expiration the cavity of the thorax is lessened, and 

 that of the abdomen is restored to its former size ; and the displaced viscera 

 return to their usual place. By the contraction of the fibres the aperture 

 for the oesophagus will be rendered smaller, and that tube may be com- 

 pressed ; but the other openings for the vena cava and aorta do not ex- 

 perience change. 



Preparatory to the making of a great muscular effort, the midriff con- 

 tracts, and descends for the purpose of permitting a full quantity of air to 

 enter the thorax. Till the effort is over the diaphragm remains in a de- 

 pressed position. Its action is commonly involuntary, but the movement 

 can be controlled by the will at any stage. 



Parts of the diaphragm. The following named parts, which have been 

 noticed shortly in describing the muscle, are now to be referred to more 

 fully, viz., the central tendon, the pillars, the arches, and the apertures. 



The central tendon (cordiform tendon) occupies the middle of the dia- 

 phragm (fig. 170), and is surrounded by muscular fibres; the large vena 

 cava pierces it. It is of a pearly white color, and its tendinous fibres 

 cross in different directions. In form it resembles a trefoil leaf: of its 

 three segments the right (c) is the largest, and the left (a) the smallest. 



The pillars (crura) are two large muscular and tendinous processes (d 

 and e), one on each side of the abdominal aorta. They are pointed 

 and tendinous below, where they are attached to the upper lumbar verte- 

 brae, but large and fleshy above ; and between them is a tendinous arch 

 over the aorta. 



In each pillar the fleshy fibres pass upwards and forwards, diverging 

 from each other : the greater number join the central tendon without in- 

 termixing, but the inner fibres of the two crura cross one another in the 

 following manner : Those of the right (e) ascend by the side of the aorta, 

 and pass to the left of the middle line decussating with the fibres of the 

 opposite crus between that vessel and the opening of the oesophagus. The 

 fibres of the other crus (c?) may be traced in the same way, to form the 

 right half of the ossophagean opening. In the decussation the fasciculus of 

 fibres from the right crus is generally larger than that from the left, and 

 is commonly anterior to it. 



The pillars differ somewhat on opposite sides. The right (e) is the 

 larger of the two, and is fixed by tendinous processes to the bodies of the 

 first three lumbar vertebrae, and their intervertebral substance, reaching 

 to the disk between the third and fourth vertebrae. The left pillar (d) 

 (sometimes absent) is situated more on the side of the spine, is partly con- 

 cealed by the aorta, and does not reach so far as the right by the depth of 

 a vertebra, or of an intervertebral substance. 



The arches (ligamenta arcuata) are two fibrous bands on each side over 



