APPENDIX. 87 



The natural order in following the course of the 

 aliment from the stomach as a guide, leads from the 

 absorbents to the heart, which in the caterpillar is a 

 simple canal or artery running along the middle of 

 the back, admitting of undulation of the blood. 

 From this simple structure it becomes, in different 

 animals, by small additions, more and more complex, 

 till it arrives at the degree of perfection which is dis- 

 played in the human heart. These are followed by 

 the different structures of valves in the arteries and 

 veins, and the coats of these vessels. Then the 

 lungs are shewn in all their gradations, from the 

 simple vascular lining of the egg-shell, which serves 

 as lungs for the chick, to those of the more per- 

 fect animals. In one instance, viz. that of the siren, 

 both gills and lungs are seen in the same animal. 

 The windpipe and larynx are then shewn, under all 

 their different forms. The kidneys make the last 

 part of this subject. 



The third class takes up the most simple state of 

 the brain, which is in the leech a single nerve with 

 ramifications. In the snail, the brain forms a circu- 

 lar nerve, through the middle of which passes the 

 oesophagus, from which circle there are branches 

 going to every part of the skin of the animal. In 

 the insect, the brain has a more compact form ; is 

 larger in fish, but still more so in birds, gradually 

 increasing in size as the animal is endowed with a 

 greater degree of sagacity, till at last it become the 

 large complex organ found in the elephant, and ia 



