THE ABDOMEN 1147 



THE ABDOMEN. 



The abdomen is the largest cavity in the body. It is of an oval shape, the extrem- 

 ities of the oval being directed upward and downward. The upper extremity is 

 formed by the diaphragm which extends as a dome over the abdomen, so that the 

 cavity extends high into the bony thorax, reaching on the right side, in the mammary 

 line, to the upper border of the fifth rib ; on the left side it falls below this level by 

 about 2.5 cm. The lower extremity is formed by the structures which clothe the 

 inner surface of the bony pelvis, principally the Levator ani and Coccygeus on 

 either side. These muscles are sometimes termed the diaphragm of the pelvis. 

 The cavity is w r ider above than below, and measures more in the vertical than in 

 the transverse diameter. In order to facilitate description, it is artificially divided 

 into two parts : an upper and larger part, the abdomen proper; and a low r er and smaller 

 part, the pelvis. These two cavities are not separated from each other, but the 

 limit between them is marked by the superior aperture of the lesser pelvis. 



The abdomen proper differs from the other great cavities of the body in being 

 bounded for the most part by muscles and fasciae, so that it can vary in capacity 

 and shape according to the condition of the viscera which it contains; but, in addi- 

 tion to this, the abdomen varies in form and extent with age and sex. In the adult 

 male, with moderate distension of the viscera, it is oval in shape, but at the same 

 time flattened from before backward. In the adult female, with a fully developed 

 pelvis, it is ovoid with the narrower pole upward, and in young children it is also 

 ovoid but with the narrower pole downward. 



Boundaries. It is bounded in front and at the sides by the abdominal muscles 

 and the Iliacus muscles; behind by the vertebral column and the Psoas and 

 Quadratus lumborum muscles; above by the diaphragm; below by the plane of 

 the superior aperture of the lesser pelvis. The muscles forming the boundaries 

 of the cavity are lined upon their inner surfaces by a layer of fascia. 



The abdomen contains the greater part of the digestive tube; some of the 

 accessory organs to digestion, viz., the liver and pancreas; the spleen, the kidneys, 

 and the suprarenal glands. Most of these structures, as well as the wall of the 

 cavity in which they are contained, are more or less covered by an extensive and 

 complicated serous membrane, the peritoneum. 



The Apertures in the Walls of the Abdomen. The apertures in the walls of the 

 abdomen, for the transmission of structures to or from it, are, in front, the umbilical 

 (in the fetus), for the transmission of the umbilical vessels, the allantois, and vitel- 

 line duct; above, the vena caval opening, for the transmission of the inferior vena 

 cava, the aortic hiatus, for the passage of the aorta, azygos vein, and thoracic duct, 

 and the esophageal hiatus, for the esophagus and vagi. Below, there are two 

 apertures on either side: one for the passage of the femoral vessels and lumbo- 

 inguinal nerve, and the other for the transmission of the spermatic cord in the male, 

 and the round ligament of the uterus in the female. 



Regions. For convenience of description of the viscera, as well as of reference 

 to the morbid conditions of the contained parts, the abdomen is artificially divided 

 into nine regions by imaginary planes, two horizontal and two sagittal, passing 

 through the cavity, the edges of the planes being indicated by lines draw r n on the 

 surface of the body. Of the horizontal planes the upper or transpyloric is indicated 

 by a line encircling the body at the level of a point midway between the jugular 

 notch and the symphysis pubis, the lower by a line carried around the trunk at the 

 level of a point midway between the transpyloric and the symphysis pubis. The 

 latter is practically the interfcubercular plane of Cunningham, who pointed out 1 

 that its level corresponds with the prominent and easily defined tubercle on the 

 iliac crest about 5 cm. behind the anterior superior iliac spine. By means of these 



1 Journal of Anatomy and Physiology, vol. xxvii. 



