ABDOMINAL CAVITY 429 



part, the pubic, ischial, sacral, and coccygeal bones form very 

 complete bony boundaries for the cavity. 



Whilst the abdominal cavity, therefore, is very fully pro- 

 tected, posteriorly and laterally, by skeletal parts, the front 

 wall is almost entirely formed by the muscles and apo- 

 neuroses which have been dissected in this region. 



From this it will be seen that the roof, floor, and the 

 greater part of the abdominal wall are composed of muscular 

 structures, the contraction of which would diminish the 

 capacity of the cavity and subject the contained viscera to 

 compression. 



Subdivision of the Abdominal Cavity. In dealing with 

 so large a cavity, and one which contains such a diversity of 

 contents, it is convenient to subdivide it into regions, in 

 order that the precise position of each viscus may be 

 accurately denned. In making the first subdivision, advan- 

 tage is taken of the brim of the true pelvis. That part of 

 the cavity which is situated above this is termed the abdomen 

 proper ; whilst the part which lies below it is called the 

 pelvic cavity. These two portions of the general abdominal 

 cavity do not lie the one directly over the other. The long 

 axis of the abdomen proper is very nearly vertical ; that of 

 the pelvic cavity is very oblique, and directed backwards and 

 downwards. Indeed, the pelvic cavity presents the appearance 

 of a recess leading backwards and downwards from the lower 

 and posterior part of the abdominal cavity (Fig. 160). 



The abdomen proper is still further subdivided by means of 

 four arbitrary planes of section. Two of these are supposed 

 to pass through the body in a horizontal direction, and two in 

 a vertical direction. The former are termed the subcostal and 

 the intertubercular planes of subdivision, and the position of 

 each is determined as follows : A horizontal line is drawn 

 around the body at a level corresponding to the most 

 dependent parts of the tenth costal arches. This gives the 

 position of the subcostal plane. A second line is drawn 

 horizontally around the trunk so as to pass, on each side, 

 through the highest point on the iliac crest that is seen, 

 from the front. This point is always easily determined, as 

 a prominent tubercle juts out from the lateral lip of the iliac 

 crest at the spot. It is situated rather more than two inches 

 behind the anterior superior iliac spine, and it marks the point 

 where the outline of the body meets the iliac crest. The 



