Chap, xvi.] THE ABDOMINAL PARIETES. 293 



If the blow be anticipated the muscles of the belly 

 will be instinctively contracted, and the viscera be at 

 once provided with a firm but elastic shield. Thus 

 the abdominal muscles have been found bruised and 

 torn while the viscera were intact, and, on the other 

 hand, in cases probably where the muscles were inert or 

 taken unawares a viscus has been found to be damaged 

 without any conspicuous lesion in the belly wall. 



Along the linea alba the abdominal wall is thin, 

 dense, and free from visible blood-vessels. Hence in 

 most operations upon the abdominal cavity the incision 

 is made in the middle line. Along the outer border of 

 the rectus muscle (i.e. about and just beyond the linea 

 semilunaris) the parietes are also thin and lacking in 

 vessels, and consequently that situation is well suited 

 for an incision. There are, however, few operative 

 circumstances except abdominal nephrectomy that are 

 met by an incision so placed. In most cases it is a 

 question of either opening the abdomen in the middle 

 line or in one of the loins. Below the navel the two 

 recti muscles are almost in contact, and here the linea 

 alba can scarcely be said to exist. Above the umbili- 

 cus the two muscles tend to separate a little, and the 

 " line " increases greatly in width. In pregnancy and 

 in other forms of distended abdomen the median 

 interval between the recti may be much increased. 



The structures immediately behind the linea alba 

 are, from above downwards, the liver, stomach, and 

 transverse colon above the umbilicus, and the small in- 

 testines and bladder (when distended) below it. There 

 are often little spaces between the fibres of the linea 

 alba, and through these pellets of fat from the sub- 

 peritoneal tissue may grow. If of fair size these little 

 masses may be mistaken for irreducible herniae. Similar 

 growths of fat may extend through the inguinal or 

 femoral openings, and constitute the so-called " fatty 

 herniae." 



