394 EXTERNAL RESPIRATION 



muscles of the abdominal wall, viz. : external oblique, internal 

 oblique, transversalis and rectus abdominis on each side. 



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 ivall zvhich ivill move forwards. Correspond- 

 ingly, 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. 



This musculature has also an important part to play in the 

 maintenance of an adequate circulation. There is no doubt 

 that the diaphragm, with its synergic and antagonistic muscles, 

 was evolved not in connection with respiration, but with circula- 

 tion. Amphibians, for instance, carry on their interchanges of 

 air between lungs and atmosphere by the action of muscles under 

 the jaw. In the mammal, without the constant tension of the 

 abdominal muscles applied to the abdominal viscera, the larger 

 veins would become distended with blood, and these veins are 

 capable of holding the entire amount of blood in the body. So, if 

 for any reason the muscles of the abdominal wall lose tone, a 

 considerable fall in arterial blood pressure is the result. It may 

 even fall to zero and death ensue. This may be determined 

 experimentally, either by dividing the spinal cord at the level of 

 the first thoracic vertebra, or by using an animal with poorly 

 developed abdominal muscles such as the tame rabbit. In the 

 first case, the influence of the bulbar centres on the part below the 

 section is removed, and the tone of the abdominal wall is abolished. 

 If the animal is now placed vertically erect, the abdominal veins 

 distend under the haemostatic pressure. In them such a large 



