20 



BRITISH MAMMALS 



and below this a tiny spur which is a rudiment of the tibia, or 

 principal bone of the leg. In the breast bone, made of a single 

 piece (to which only one pair of ribs is attached), in the perfectly 

 symmetrical shape of the skull (both sides being absolutely alike), 

 and in the development of whalebone in the mouth, the Mystacoceti 

 differ from the toothed whales. Perhaps their nearest allies in 

 this group are the Ziphioids. 



In all whales the mammae, or mammary glands, are a single 

 pair. They are situated in the inguinal region, close to the 

 vent, and the nipple is concealed in a deep cleft. 



All that remains of the Whale's Hind Limbs [Balana). 

 From a specimen in Museum of College of Surgeons. 



Whales are wonderful instances of successful adaptation to 

 a special mode of existence. Originally land animals, they can 

 now exist with comfort under the water at a considerable depth 

 and at very low temperatures. The elaborate network of blood- 

 vessels utilises slowly and with economy the supply of oxygen 

 stored in the lungs ; the thick padding of blubber all round the 

 body is an elastic cushion which protects the whale from pressure 

 at great depths or from the effects of cold ; so that whales can 

 pass with indifference from the Tropic to the Arctic seas. Blubber 

 is fat (sometimes liquid) held in a dense mesh of areolar tissue. 



Owing to their exclusively aquatic existence, and almost 

 equally exclusive confinement in these latitudes to salt water, it 



