268 SURGICAL APPLIED ANATOMY. [Chap. xvi. 



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 that could be 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 hence the white 

 line is very narrow. Above the umbilicus 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 

 intestines 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 subperitoneal tissue may grow. If of fair size these 

 little masses may be mistaken for irreducible hernise. 



The fibrous ring of the umbilicus is derived from 

 the linea alba. To this ring the adjacent structures, 



