43 



ANIMAL RESPIRATION 



subdivisions of the Mammals, but the order (Cetacea) which 

 includes Whales, Porpoises, &c., is of special interest in this 

 connection, and therefore merits a brief notice. Although these 

 animals are obliged to come frequently to the surface in order to 

 breathe or " blow ", they can remain under water without incon- 

 venience for between three and four minutes. It is not, therefore, 

 surprising to find that their lungs are of large size, and it may also 

 be noted that parts of the body-wall are richly supplied with ela- 

 borate net-works of blood-vessels aggregated into thick masses 

 (" wonder-nets ") which possibly serve as a means of storing 

 purified blood, though this is only a conjecture. The midriff is 

 very thick, and its central tendon comparatively small, and the 

 unusually large amount of muscle present renders the movements 

 of breathing particularly vigorous. The most remarkable pecu- 

 liarity, however, is the very complete separation which takes 

 place between the breathing and feeding tracts, pretty much 

 as in a Crocodile (see p. 425), the object being similar, i.e. to 

 enable the animal to open its mouth under water without risk 

 of choking from entry of fluid into the air-tubes. The top of 

 the windpipe is a projecting cone, the end of which fits closely 

 into the back of the nasal passage, leaving, however, a space on 

 either side by which food can travel on into the gullet. In order 

 that the animal may breathe with as small a part of the body out 

 of water as possible the nostrils are shifted to the top of the head, 

 and are very close together, or even fused into one, constituting two 

 or one "blowhole", as the case may be. When in the colder ocean 

 regions a whale comes to the surface and breathes out air from its 

 lungs, the watery vapour with which this is abundantly charged is 

 condensed by the cold so as to become visible. This appearance 

 has given rise to the common but erroneous idea that a whale 

 " spouts " out of its blowhole the water that has been taken in at 

 the mouth. Such a procedure would be impossible for anatomical 

 reasons, and the superfluous water which is taken in with the 

 minute animals that mostly constitute the food (see p. 29) really 

 passes out at the sides of the mouth. 



In the newly-born young of Pouched Mammals (Marsupials) 

 such as Kangaroos, the breathing and feeding tracts are separated 

 in much the same way as in the Whales and their allies, but for a 

 rather different reason. Young Marsupials when they first come 

 into the world are in an exceedingly immature and helpless con- 



