RESPIRATION IN xMAMMALIA. 293 



arches of the ribs, which extend from the spine to 

 the sternum, or breast-bone, and produce mecha- 

 nical support on the same principle that a cask is 

 strengthened by being girt with hoops, which, 

 though composed of comparatively weak materials, 

 are yet capable, from their circular shape, of pre- 

 senting great resistance to any compressing force. 

 While Nature has thus guarded the chest with 

 such peculiar solicitude, against the efforts of any 

 external force tending to diminish its capacity, 

 she has made ample provision for enlarging or 

 contracting its diameter in the act of respiration. 

 First, at the lower part, or that which corresponds 

 to the base of the cone, the only side, indeed, 

 which is not protected by bone, there is extended 

 a thin expansion, partly muscular, and partly ten- 

 dinous, forming a complete partition, and closing 

 the cavity of the chest on the side next to the ab- 

 domen. This nmscle is called the Diaphmgni : it 

 is perforated, close to its origin from the spine, by 

 four tubes, namely, the oesophagus, the aorta, the 

 vena cava, and the thoracic duct. Its surface is not 

 flat, but convex above, or towards the chest ; and 

 the direction of its fibres is such that when they 

 contract they bring down the middle part, which is 

 tendinous, and render it more flat than before, (the 

 passage of the four tubes already mentioned, not 

 interfering with this action,) and thus the cavity of 

 the thorax may be considerably enlarged. It is 

 obvious that if, upon the descent of the diaphragm, 

 the lungs were to remain in their original situation, 

 an empty space woidd be left between them and 

 the diaphragm. But no vacuum can take place in 

 the body; the air cells of the lungs must always 



