THE ABDOMINAL CAVITY. 1157 



'posteriorly, in which latter position the cavity is further protected by the vertebral 

 solumn. Anteriorly, on the other hand, the ribs are wanting below the sternum, 

 and there the abdominal wall is formed only of aponeuroses and muscles. But even 

 at the sides and back there is a considerable zone, usually one to two inches wide, 

 between the lower ribs above and the crest of the ilium below, which has no 

 bony support except that afforded by the vertebral column. 



Whilst the circumference of the diaphragm is attached to the inferior part of 

 the thoracic framework anteriorly and laterally, and to the lumbar vertebrae 

 ; ; posteriorly, the central portion of the dome, on the other hand, namely, the central 

 tendon, is placed high up, under cover of the ribs, and in a more or less horizontal 

 plane. As a result, the peripheral muscular part slopes upwards and medially from the 

 3ircumference of the thoracic framework to the central tendon, and lies for a con- 

 siderable distance in contact with the deep surface of the ribs ; thus the diaphragm 

 somes to form, not only the roof of. the cavity, but it also enters into the formation 

 of the sides, the posterior wall, and, to a less extent, of the anterior wall ; and almost 

 %s much of the cavity of the abdomen as of the thorax lies under shelter of the ribs. 



Owing to the fact that the boundaries of the abdomen are formed chiefly of 

 muscles, it follows that its walls are capable of contraction to a very considerable 

 3xtent, and the size of the cavity can consequently be altered in all directions. Its 

 3hief changes in form are due to the descent or elevation of the diaphragm, the 

 contraction or relaxation of the anterior wall and the side walls, and the raising 

 IT lowering of the pelvic floor. 



The superior aperture of the pelvis minor (Figs. 234 and 235, p. 236), which 

 separates the two natural divisions of the cavity, is formed behind by the base of 

 the sacrum, at the sides by the linea terminalis of each hip bone, and in front 

 by the pubic crests and the symphysis pubis. In the erect position it usually 

 makes an angle of about 55 to 60 degrees with the horizontal. The two portions 

 }f the abdominal cavity which the superior aperture separates meet at an angle, 

 the abdomen proper extending almost vertically upwards from it, whilst the pelvic 

 cavity slopes backwards and slightly downwards. 



The pelvic cavity is bounded in front and at the sides by the portions of the 

 lip bones below the level of the linea terminalis. Those portions of the bony wall are 

 oartly clothed by the obturator internus muscles, and, internal to those muscles, by the 

 >arietal portion of the pelvic fascia, as low down as the arcus tendineus. The posterior 

 \VB\\ is formed by the pelvic surface of the sacrum, covered on each side by the piriformis 

 muscle. That wall (as represented by the piriformes muscles) meets the side wall 

 it the anterior border of the greater sciatic foramen ; through that foramen the piriformis 

 passes out, thus closing up what would otherwise be a large aperture in the parietes of 

 :he cavity. The floor is composed of the two pairs of muscles which form the pelvic 

 iiaphragm, namely, the levatores ani and the coccygei covered by the visceral layer of the 

 3iidopelvic' fascia. Those muscles pass, on each side, from the side wall of the pelvis, 

 iownwards and medially towards the median plane, and present a concave superior surface 

 bowards the pelvic cavity. 



Within the muscles forming its walls, the abdomen is lined by an envelope of 

 fascia, which separates the muscles from the extraperitoneal connective tissue and 

 peritoneum. That fascial layer is distinguished in different localities as: (1) 

 the fascia transversalis, on the anterior wall and the side walls, lining the deep 

 surface of the transversalis muscle and continuous above with the fascia clothing 

 the inferior surface of the diaphragm; (2) the fascia iliaca, on the posterior wall, 

 3overing the psoas and iliacus muscles ; (3) the fascia diaphragmatica, covering the 

 inferior surface of the diaphragm ; and (4) the fascia endopelvina, lining the pelvis. 



Apertures. Certain apertures are found in the walls of the abdomen, some of 

 which lead to a weakening of the parietes. They are: the three openings in the 

 diaphragm for the passage of the inferior vena cava, the oesophagus, and the aorta, 

 respectively; the apertures in the pelvic floor, through which the rectum, the 

 urethra, and the vagina in the female, reach the surface ; the inguinal canal, 

 through which the spermatic funiculus (or the round ligament) passes, in leaving the 

 abdominal cavity ; and lastly, the femoral canal, a small passage which extends down- 

 wards from the abdomen along the medial side of the femoral vessels. The latter two, 



