RESPIRATION IN MAMMALIA. 325 



the continuance of life being essentially depen- 

 dent on its vigorous and unceasing exercise. 

 The whole class of Mammalia have lungs of an 

 exceedingly developed structure, composed of 

 an immense number of minute cells, crowded 

 together as closely as possible, and presenting a 

 vast extent of internal surface. The thorax, or 

 cavity in which the lungs, together with the 

 heart and its great blood-vessels, are inclosed, 

 has somewhat the shape of a cone ; and its sides 

 are defended from compression by the arches of 

 the rib3, which extend from the spine to the 

 sternum, or breast-bone, and produce mechani- 

 cal support on the same principle that a cask is 

 strengthened by being girt with hoops, which, 

 though composed of comparatively weak mate- 

 rials, are yet capable, from their circular shape, 

 of presenting great resistance to any compress- 

 ing force. 



While Nature has thus guarded the chest, with 

 such peculiar solicitude, against the efforts of 

 any external force, tending to diminish its capa- 

 city, she has made ample provision for enlarging 

 or contracting its diameter in the act of respira- 

 tion. First, at the lower part, or that which 

 corresponds to the basis of the cone, the only 

 side, indeed, which is not defended by bone, 

 there is extended a thin expansion, partly mus- 

 cular, and partly tendinous, forming a complete 

 partition, and closing the cavity of the chest on 



