ABDOMINAL MUSCLES 305 



the lower rib are attached dorsally to the anterior half of the crest 

 of the ileum. The middle and upper parts of each muscle termi- 

 nate in aponeuroses which, covering the whole of the front of the 

 abdomen, are joined together at the mid line (linea alba). 



(b) Internal Oblique (Ascendens). This pair of muscles lies 

 between the external oblique and the transversalis and is attached 

 below to the outer half of Poupart's ligament and to the anterior 

 two-thirds of the crest of the ileum. Dorsally it is attached to 

 the lumbar fascia. From this origin, the fibres spread in a fan- 

 shape passing behind to the lower three ribs and in front forming 

 an aponeurosis. 



(c) Transversalis. A pair of flat muscles lying behind the other 

 pairs. The fibres arise from the outer third of Poupart's ligament 

 and from the anterior two-thirds of the iliac crest. They are 

 attached behind to the lumbar fascia and above to the inner sur- 

 faces of the cartilages of the lower six ribs. The fibres run 

 transversely and horizontally, being inserted into the aponeurosis 

 of the linea alba. 



(d) Rectus. This muscle extends along the front of the abdomen 

 on each side of the linea alba from the sternum to the pubes. 



The floating ribs (and in 40 per cent, of people, the tenth rib 

 also) are functionally part of the abdominal wall. Their move- 

 ments are controlled by the quadratus lumborum and erector 

 spinae muscles. The twelfth rib, in addition, is anchored to the 

 transverse processes of the first and second lumbar vertebrae by 

 a strong ligamentous membrane, an extension of the middle layer 

 of the lumbar fascia. In this way the upward movement of the 

 rib especially in its spiral segment is restricted. The anterior 

 and lateral segments have a freer movement, so permitting of a 

 movement of the floating ribs (and the tenth) round an axis corre- 

 sponding to their spinal segments. It has been noticed that 

 during inspiration, the spaces between those ribs widen and that 

 during expiration the reverse takes place. 



Function of Abdominal Muscles. The four pairs of abdominal 

 muscles and their fibrous attachments act antagonistically to 

 the diaphragm. When the latter contracts the former have to 

 yield to accommodate the displaced . viscera. That is, during 

 diaphragmatic breathing, inspiration is accompanied by a relaxa- 

 tion of the abdominal wall which will move forwards. Corre- 

 spondingly, expiration will be aided by the tendency of the 

 viscera to return to their normal positions and by the return of 

 the abdominal muscles to the position of rest. 



B. B. 20 



