264 ABDOMEN 



wall is almost entirely formed by the muscles and aponeuroses 

 which have already been dissected. 



It is obvious, however, that the roof, floor, and the 

 greater part of the abdominal wall are composed of muscular 

 structures, the contraction of which diminishes the capacity 

 of the cavity and subjects the contained viscera to com- 

 pression. 



Subdivision of the Abdominal Cavity. In order that the 

 exact positions of the numerous and diverse contents of the 

 abdomen may be accurately defined it is necessary to divide 

 the cavity into regions. In the first place the cavity of the 

 abdomen is divided into two principal parts, the abdomen 

 proper and the pelvis minor. The plane of separation 

 between the two is an imaginary plane which lies at the 

 level of the upper aperture of the pelvis minor, extending 

 from the front of the promontory of the sacrum to the upper 

 border of the symphysis pubis. The two main parts of the 

 cavity are not in direct line with one another. The long 

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

 minor is very oblique, and is directed backwards and down- 

 wards. The difference of direction is so great (see Fig. 117), 

 and the difference in size is so marked, that the pelvis minor 

 has the appearance of a large recess which projects backwards 

 and downwards from the lower part of the abdomen proper. 



The abdomen proper is still further subdivided by means 

 of four arbitrary planes of section. Two of these 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, drawn 

 around the body at the level of the most dependent parts of 

 the tenth costal arches, indicates the position of the subcostal 

 plane. A second line, drawn around the body at the level of 

 the highest parts of the iliac crests, which can be seen from 

 the front, gives the level of the intertubercular plane. 



The highest part of each iliac crest which can be seen from the front 

 lies about 50-60 mm. (two to two and a half inches) behind the anterior 

 superior spine of the ilium. Its position is indicated by the junction of 

 the lateral outline of the trunk with the outline of the hip bone at the 

 point where a prominent tubercle juts out from the external lip of the 

 iliac crest ; hence the term intertubercular plane. 



The two horizontal planes map out the abdomen into 



