306 EXTERNAL RESPIRATION 



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. Without the constant tension of the abdominal muscles 

 applied to the abdominal viscera, the larger veins would become 

 distended with blood. These veins are capable of holding the 

 entire amount of blood in the body. If for any reason the muscles 

 of the abdominal wall lose tone, 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 dorsal 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 verti- 

 cally erect, the abdominal veins distend under the haemostatic 

 pressure. In them the whole of the blood collects and there is 

 no blood to fill the heart. 



III. Thoracic Respiration. The upper and lower regions of the 

 thorax should be considered separately. The muscles and move- 

 ments of the upper series of ribs are quite different from the lower 

 series. 



(a) Lower costal series (6th to 9th or 10th rib). 



This segment moves along with the diaphragm and leads to 

 the expansion downwards of the lower lobes of the lungs. The 

 ribs are articulated to the spinal column so that during inspira- 

 tion the lateral and anterior part of each moves outward more 

 than the one above it. Two movements may be noted : 



(i) The 50 to 7 mm. of each rib next the spine to which is 

 attached the erector spinae muscle moves forward at each respira- 

 tion. The tubercle of the rib slides forward on the flat upper 

 facet of the transverse process. 



(ii) The non-spinal portion of a pair of ribs moves with a bucket- 

 handle action, rising and coming forward with each inspiration. 

 At the centre of each pair is the sternum-cartilage complex which 

 is raised and forced forwards during inspiration. The muscles 

 concerned in this increase of the volume of the lower thorax, 

 transversely and antero-posteriorly, are the external intercostals. 



(/;) Upper costal series (2nd-5th rib). 



