220 OUTLINES OF EQUINE ANATOMY. 



part of tlie posterior third of tlie linea alba is the umbi- 

 licus ; this is a slight puckering of the layers forming the 

 iloor of the abdomen to close ujd an opening which, in the 

 foetus, admitted the passage of the umbilical arteries and 

 the urachus to the foetal membranes, and the umbilical 

 veins from them ; it becomes impervious in extra-uterine life, 

 being no longer needed. 



Obliquus abdominis externus is attached to the external 

 surface of the fourteen posterior ribs almost as low down 

 as their cartilages, having anteriorly four fleshy digitations 

 with serratus magnus. From these points its fibres run 

 •obliquely downwards and backwards to the under surface 

 of the ensiform cartilage, the linea alba, symphysis pubis, 

 antero-inferior spinous process of the ilium and lumbar 

 faschia. Some of the fibres are arranged to form Poupart's 

 ligament (vide supra) into which others are inserted, while 

 some run to become attached by aponeurosis to the internal 

 . surface of the hind limb. Removing this muscle we expose 

 some thin tendinous fibres running obliquely downwards 

 and forwards ; these belong to 



Obliquus abdominis internus, which seems to be composed 

 of fibres radiating from the antero-inferior spinous process 

 of the ilium and the lumbar faschia and the transverse 

 processes of the three anterior lumbar vertebrae. The most 

 superior of these run to the last rib, and being slightly 

 separated from the rest are sometimes considered as a dis- 

 tinct muscle and termed retractor costae ; a few run to the 

 internal surface of the posterior cartilages of the false ribs, 

 most of them running to the linea alba along its whole 

 length, the posterior ones separating to form the internal 

 abdominal ring and the inner wall of the inguinal canal. 



Extending for a considerable distance on either side of 

 the linea alba, forming the under part of the wall of the 

 belly, we see a muscle wide centrally and growing narrow 

 ■anteriorly to become attached to the under surface of the 

 ■ ensiform cartilage and to the external surface of the four 

 posterior sterno-costal cartilages of the true ribs. Poste- 

 riorly it grows narrow to become attached to the thick tendon 

 running to the anterior part of symphysis pubis ; its inner 

 margin is attached along the whole length of linea alba, 

 and it presents many transverse tendinous markings, 

 by which it is considerably strengthened ; this is rectus 

 .abdominis, its internal surface lies in contact with a few 



