54 
Dr. Harwood on a 
of the sturgeon kind, which has so large and free a commu- 
nication with the oesophagus ; but in extending our enquiries 
on this subject further, we find no class among the vertebral 
animals which does not offer some modification of a similar 
structure ; for even among the mammalia, the whales, which 
are called the baleenae boops and rostrata, also the balaena 
musculus, if it be a distinct species, have the anterior part of 
the body covered with an extremely elastic skin, plaited into 
deep folds, as Mr. Hunter has expressed it, “ like unto a 
ribbed stocking/’ and capable of a vast extent of dilatation. 
This is partly exhibited in the act of swallowing food, and 
appears reasonably intended, by communicating with the 
external air, to render the body so specifically light, as to 
enable them to sleep with greater ease upon the surface. Of 
this communication however, that accurate observer, Mr. 
Hunter, has taken no notice, but adds, “ why the skin should 
be so elastic, is difficult to say, as it covers the thorax, which 
can never be increased in size ; yet there must be some pecu- 
liar circumstance in the economy of the species requiring this 
structure, which we as yet know nothing of.” Among birds, 
several examples occur of the possession of external air 
vessels of considerable size, which in like manner appear to 
be intended to more perfectly increase or diminish their spe- 
cific gravity at their will. Of this structure the gigantic stork, 
ardea argala Lin. presents a striking example. Lastly, among 
reptiles, such mechanical aids, either internal or external, are 
far more numerous, for to this end we must consider the 
large internal vesicle forming an appendage to the lungs of 
turtles ; and probably to the same end is the saccular and 
membranous inferior half of the lung in snakes, which is at 
