322 MUSCLES OF THE ABDOMEN. 



of some of the lower animals ; they are rather vestiges of the septa between the 

 original vertebral myotomes. They sometimes extend outwards from the rectus, 

 and penetrate partially into the internal oblique. 



The pyrainidalis is a small muscle resting on the lower part of the 

 rectus. It arises from the frout of the pulns and the ligaments of the 

 symphysis, and becoming narrow as it ascends over the lower third of 

 the interval between the umbilicus and })ubis, is inserted into the linea 

 alba. 



lielatlons. — Tlie p;yTamidalis is covered in front by the aponeurosis of the other 

 muscles, and rests posteriorly on the rectus, the size of the lower part of which 

 is augmented when the pyramidalis is wanting. 



Varlrtli's. — This muscle is often abseiit on one or both sides : in some instances 

 it has been found to be double. It occasionally exceeds the length above stated. 



The linoa alia is a white fibrous structure, extended perpendicularly 

 downwards in the middle line from the cusiform cartilage to the pubis. 

 This tendinous l)and is formed by the union of the aponeuroses of the 

 two oblifpie and the transverse muscles, the tendinous fibres being con- 

 tinued in a decussating manner from one side to the other. Some 

 longitudinal filires are distinguishable towai-ds its lower end. It is 

 broader superiorly than inferiorly, and a little below the middle is 

 ■widened out into a circular flat space, in the centre of which is situated 

 the cicatrix of tlic nmhilkus. 



The li)ic(C. somilimares are the two curved linear spaces on the surface 

 of the abdomen, placed externally to the outer margins of the recti 

 muscles. They are produced by the union of the aponeurotic tendons 

 of the oblique and transversalis muscles, and they correspond on their 

 inner side to the outer margin of the sheath of the rectus. 



The quadratus lumborum is an irregularly quadrilateral muscle, 

 slightly broader below than above, placed between the last rib and the 

 crest of the ilium, close to the vertebral column. It is divisible into 

 two parts. One of these, arising by fleshy and tendinous fibres from the 

 ilio-lumbar ligament, and from the iliac crest for several inches near the 

 place where that ligament is attached, is inserted into the inferior 

 border of the last rib for about half its length, and by four tendinous 

 slips into the transverse processes of the four superior lumbar vertebra. 

 Another series of fibres, arising by two or three tendinous slips from as 

 many of the inferior transverse processes at their upper margins, passes 

 in front of those inserted into the same processes, and joins with the 

 part of the muscle attached to the rib. 



Jichifions. — This muscle is placed between the middle and deepest layer of the 

 lumliar aponeurosis, and its inner part is covered in front by the psoas muscle. 



^'(ll•k'fk^K. — The number of the points of insertion of this muscle to the ver- 

 tebrae, and the extent of its connection with the last rib. vary in different 

 instances. It is sometimes attached to the body or transverse process of the last 

 dorsal vertebra. 



Actions. — The abdominal muscles not only form a great part of the wall to 

 enclose and support the abdominal viscera, but by their contractions are capable 

 of acting successively on those viscera, on the thorax, and on the pelvis. "When 

 the pelvis and thorax are fixed, the abdominal muscles constrict the cavity and 

 compress the viscera, particularly if tlie diajihragm be fixed or be made to descend 

 at the same time, as occurs in vomiting and in the expidsion of the foetus, the 

 fajccs, and the urine. 



