INTRODUCTION. 25 



ing characters of animals in general, that is, their 

 intestinal cavity, from which, passing through the 

 pores and vessels, which may be considered a kind 

 of internal roots, the nutritive fluid penetrates every 

 part of their system, and sustains the whole. 



The organization of this cavity and its appurte- 

 nances must, of necessity, vary and correspond with 

 the nature and kind of aliment on which each spe- 

 cies subsists, and the changes such animals must 

 undergo to convert them into juices fit for the pur 

 pose of absorption. Plants, on the other hand, are 

 supplied by the soil and the surrounding atmosphere 

 with juices already prepared for absorption. The 

 animal, whose functions are so much more numerous 

 and varied than those of the plant, must consequently 

 possess an organization so much the more compli- 

 cated; and as from the nature of its constitution its 

 parts could not preserve one fixed position rela- 

 tively to each other, it is obvious that the motion of 

 their fluids could not be produced by external causes, 

 but must be, in a great measure, independent of 

 atmospheric influence ; here we trace the second 

 leading distinction, or important character, of the 

 animal world, namely, the circulating system, which 

 is less essential than the digestive, and is unneces- 

 sary in animals of the simpler conformation. The 

 animal functions requiring certain organic systems, 

 not requisite in plants, the muscular system, for 

 instance, for the purpose of voluntary motion, and 

 the nervous for sensibility ; and these systems, like 

 all the rest, acting through the medium of the fluids, 



