428 



ANIMAL RESPIRATION 



and Mammals form a series of increasing complexity in this respect, 

 keeping pace, as it were, with increasing need for rapid oxygenation 

 of the blood, which is most marked in the highest animals. An- 

 other series is constituted by the breathing organs of Amphibians, 



Reptiles, and Birds, culminat- 

 ing in the last, which surpass 

 even Mammals as regards re- 

 spiratory efficiency. They 

 have, however, as we have 

 seen, specialized on rather dif- 

 ferent lines. 



As to the mechanical ar- 

 rangements by means of which 

 air is renewed in the lun^s of 

 Mammals, the movement of 

 ribs and breastbone, so as to 

 alternately increase and di- 

 minish the size of the thorax, 

 takes place in much the same 

 way as in Reptiles and Birds. 

 There is, besides, a fresh factor 

 of great importance in the 

 midriff or diaphragm, a parti- 

 tion by which the cavity of the thorax is separated from that 

 of the abdomen. This may be described as a thin, curved 

 muscle, convex towards the thorax, and possessing a central ten- 

 dinous part. There are also some strong bundles of muscle, 

 the " pillars " of the diaphragm, which run from the under side 

 of the backbone in the region of the loins to the part of the 

 midriff furthest from the breastbone. The volume of the thorax is 

 increased in the direction of its length, and the taking in of air 

 promoted, by the contraction of the muscular margin of the midriff 

 and of its pillars, so that the edge of this partition becomes flat- 

 tened. When the muscle ceases to contract, the midriff once more 

 becomes convex towards the thorax, which therefore diminishes in 

 volume, as a result of which expulsion of impure air from the 

 lungs is greatly helped. Indeed the midriff is of the very greatest 

 importance as a breathing muscle. Backboned air-breathers which 

 are lower in the scale than Mammals possess an ill-developed 

 equivalent, though this usually has nothing to do with breathing. 



Fig. 553. — Air-passages of Lungs of Man 



A, Windpipe: B B, bronchi into which this forks; D D, smaller 

 air-tubes 



