804 ORGANS OP DIGESTION. 



plants. The alimentary surface of the plant is the exterior 

 of its root, ramified and fixed in the soil which affords it 

 food, so that a vegetable is like an animal with its stomach 

 turned inside out. As the organs of relation are those most 

 immediately connected with the varying external circumstan- 

 ces of animals, they are the most variable in their character 

 and inconstant in their existence ; but those of vegetative or 

 organic life relating to the more common and necessary 

 functions of assimilation are much more regular and constant 

 in their character. No organ, indeed, is more universal or 

 essential in animal bodies than that internal digestive cavity 

 by which they differ so remarkably from the species of the 

 vegetable kingdom. This internal sac is but an extension of 

 the primitive absorbent surface of the skin, which, in 

 animals, passes into or through the homogeneous cellular 

 tissue of the body. And, although in the simplest forms of 

 animals, this primitive sac performs alone all the assimilative 

 functions, we find it, as we ascend in the scale, giving 

 origin to various other organic systems, to which distinct 

 parts of the complex function of assimilation are successively 

 entrusted. Thus the peripheral nutrition of the plant passes 

 gradually into the central mode of the animal, and the 

 organs of organic or vegetative life, whether they open 

 internally into the digestive cavity, or on the mucous surface 

 of the skin, may be considered as originating from the 

 exterior integument, which is itself only a portion of the 

 primitive cellular tissue of the body, modified by the 

 stimulating contact of the surrounding element. As the 

 various tubular prolongations of the alimentary canal become 

 more and more developed and isolated from their primitive 

 source, they assume properties and functions more and 

 more peculiar and distinct, and thus form the numerous 

 follicular and conglomerate glandular apparatus, and the 

 various vascular systems, of animals. An alimentary cavity 

 is observed in every class of animals and almost in every 

 species, and its form and structure vary according to the 

 situation of animals in the scale, or according to the kind 

 of food on which they are destined to subsist, and the extent 

 of elaboration it requires to undergo to assimilate it to the 

 animal's body. The peculiarities presented by the digestive 

 organs are, therefore, intimately connected with the diversi- 



