ill 



situation but at the top of their heads ; so that 

 the spiracles or air-holes continue above water, 

 when all the rest of the animal is below it, and in 

 this way, like the prolonged tubes in some water- 

 insects, and the elevated wind-pipe in some water- 

 serpents, keep up respiration. Further, many of 

 these tribes, as the spermaceti whale, have large 

 cavities in their skulls filled with liquid fat > 

 which, being lighter than water, enables them, 

 without any effort, to keep a part of their heads 

 constantly above the surface. When these ani- 

 mals descend entirely into the water, the entrance 

 of this fluid into their lungs is prevented by a 

 valve, with which the spiracles are furnished, and 

 which, while it prohibits the ingress of water, 

 allows of the free egress of that received by the 

 mouth ; and it is in this way that these remark- 

 able fountains are formed which characterize the 

 animals in question. They cannot, however, 

 continue long under water ; and it is from the 

 necessity which they are under of rising fre- 

 quently to the surface for fresh air, that we are 

 enabled to capture them. Mammiferous animals, 

 like birds, inspire by enlarging the cavity in which 

 the lungs are contained ; but this, in the former, 

 is the chest alone, which is now for the first 

 time, separated from the abdomen by a proper 

 midriff. This organ is, moreover, in them a 

 principal part of the respiratory apparatus ; for, 

 while the chest is rendered broader and deeper 

 owing to the projection forwards of the breast- 

 bone by the motion of the elastic ribs, it is ren- 



