SOME INTERNAL STRUCTURES 57 



in consequence of altered habitat. The lungs them- 

 selves are characterised by their simple form. In the 

 mammalia generally the lungs are more complex. 

 They are divided into a number of separate lobes, 

 the practical result of which is to increase the lung 

 surface, without any corresponding need for an 

 enlarged chest cavity to contain them in. The same 

 result is brought about in the whale by the increased 

 length of the lungs themselves. As already mentioned 

 the chest cavity is proportionately larger than in 

 terrestrial mammals ; therefore it follows that the 

 lungs can be bigger without any lobulation. As a 

 matter of fact they are. What is uncertain at present 

 is whether the simplicity is a primitive feature in the 

 organisation of these animals, or whether it is a 

 reduction following upon the alteration of other con- 

 ditions. It is exceedingly difficult to decide such 

 matters. But before we attempt to decide, another 

 important feature of the structure of these aquatic 

 mammals must be mentioned. In many parts of the 

 body of whales the blood vessels form to a very 

 copious degree the anastomosing networks which are 

 known technically as " retia mirabilia." A rete 

 mirabile is produced by the breaking up of an artery 

 into a meshwork of minuter arterioles. The net 

 physiological result, so far as concerns the mechanical 

 effects of such a breaking up, is the slowing of the 

 blood stream at such spots, and the increase of the 

 surface of blood exposed to the surrounding organs 

 and tissues. It seems to follow from this that the 

 oxygen contained in the blood would be more fully 



