APPENDIX. 
87 
The natural order iu following the course of the 
®linient from the stomach as a guide, leads fi'om the 
absorbents to the heart, which in the caterpillar is a 
a*®ple canal or ai’teiy running along the middle of 
the back, admitting of undulation of the blood. 
Trom this simple structure it becomes, in different 
animals, by small additions, more and more complex, 
’■>11 it arrives at the degree of perfection which is dis- 
P%ed in the human heart. These are followed by 
•be different structures of valves in the arteries and 
''ains, and the coats of these vessels. Then the 
*angs are shewn in all their gradations, from the 
a'niple vascular lining of the egg-shell, which serves 
aa lungs for the chick, to those of the more per- 
j^®nt animals. In one instance, viz. that of the siren, 
“•b giiig jangg are seen in the same animal, 
he windpipe and larynx are then shewn, under all 
•heir different forms. The kidneys make the last 
of tliia subject. 
The third class takes up the most simple state of 
•be brain, which is in the leech a single nerve with 
•^•nifications. In the snail, the brain forms a circu- 
nerve, through the middle of which passes the 
•®®opliagus, from which circle there are branches 
to every part of the skin of the animal. In 
be insect, the brain has a more compact form ; is 
.®''8er in jjnt gtill more so in birds, gradually 
bteteasing in size as the animal is endowed with a 
St'eater degree of sagacity, till at last it becomes the 
'Se complex organ found in the elephant, and in 
