528 APPENDIX. 



across, between the two external ones above de- 

 scribed, the length being in proportion to the 

 thickness of the tail : a structure which gives 

 amazing strength to this part. 



The substance of the tail is so firm and com- 

 pact, that the vessels retain their dilated state, 

 even when cut across, and this section consists of 

 a large vessel surrounded by as many small ones 

 as can come in contact with its external surface ; 

 which of these are arteries and which veins I do 

 not know. 



The fins are merely covered with a strong, con- 

 densed, adipose membrane. 



The fat of this order of animals, except the 

 spermaceti, is what we generally term oil. It 

 does not coagulate in our atmosphere, and is pro- 

 bably the most fluid of animal fats. The fat is 

 differently situated in different orders of animals ; 

 in those which are the subject of the present 

 paper it is found principally on the outside of 

 the muscles, immediately under the skin, and 

 is in considerable quantity : it is rarely to be 

 met with in the interstices of the muscles, or 

 in any of the cavities, such as the abdomen, or 

 about the heart : the small quantity found in the 

 cavities of the body and interstices of parts is in 

 general disposed in the same way as in quadrupeds ; 

 but the external, which includes the principal 

 part, is inclosed in a reticular membrane, appa- 

 rently composed of fibres passing in all directions, 

 which seem to confine its extent, allowing it little 

 or no motion on itself; the whole, when distended, 



