414 Mr. Hunter’s Obfervations on the 
are in larger proportion. I will not pretend to determine how far 
this may aflift in keeping up the animal heat; but as thele ani- 
mals may be laid to live in a very cold climate or atmofphere, 
and fuch as readily carries off heat from the body, they may 
want fome help of this kind. 
It is certain that the quantity of blood in this tribe and in 
the Seal is comparatively larger than in the quadruped, and 
therefore probably amounts to more than that of any other 
known animal. 
This tribe differs from fifh in having the red blood carried 
to the extreme parts of the body, limilar to the quadruped. 
The cavity of the thorax is compofed of nearly the fame 
parts as in the quadruped; but there appears to be fome dif- 
ference, and the varieties in the different genera are greater. 
The general cavity is divided into two, as in the quadruped, 
by the heart and mediaftinum. \ 
The heart in this tribe, and in the Seal, is probably larger 
in proportion to their lize than in the quadruped, as all'o the 
blood* vefiels, more efpecially the veins. 
The heart is inclofed in its pericardium, which is attached by 
a broad furface to the diaphragm, as in the human body. It is 
compofed of four cavities two auricles, and two ventricles : 
it is more flat than in the quadruped, and adapted to the lhape 
of the cheft. The auricles have more fafciculae, and thefe 
pafs more acrofs the cavity from fide to fide than in many other 
animals ; befldes, being very mufcular, they are very elaftic, 
* As the circulation is a permanent part of the conftitution refpe&ing the 
clafs to which the animal belongs, and as the kind of heart correfponds with the 
circulation, thefe fhould be confidered in the clailing of animals. Thus we have 
animals whofe hearts have only one cavity, others with two, three, and four 
verities. 
for 
7 
