252 DIGESTION. 



the organs of absorption, must necessarily stand in a far closer 

 relation to one another than has generally been supposed to be the 

 case. If, for instance, we attempt to classify the articles of food 

 according to the manner in which they are absorbed in the intes- 

 tinal canal, such an arrangement must coincide pretty closely with 

 the changes which the different articles of diet undergo through the 

 different digestive fluids. Hence it would be by no means an 

 irrational proceeding, if we divided the different articles of food, 

 first, into such as are introduced in a state of solution into the 

 intestinal canal, and consequently are at once diffused and distri- 

 buted generally through the animal juices ; secondly, into such as 

 are rendered soluble by the digestive fluids, and in this condition 

 are, like the former, more or less diffusible ; and lastly, into such 

 as, either dissolved or undissolved, must be first metamorphosed 

 by certain digestive fluids, and even if soluble, do not undergo a 

 simple diffusion, but are conveyed by some special routes into the 

 blood and the body generally by routes on which they undergo 

 certain, although perhaps small changes before their entrance into 

 the blood. 



If, therefore, we would study the digestive process in the more 

 highly organised animals, and would not merely consider the food 

 in connection with the juices to whose digestive action it is 

 submitted, but also in reference to its passage into the blood, we 

 must especially take into consideration the organs of digestion, 

 and the laws or conditions under which absorption proceeds. If 

 further it would appear that the mechanical arrangements which 

 are exhibited in the organisms of the higher animals for the pur- 

 pose of aiding the chemical actions of the digestive fluids, and of 

 promoting the transition of the materials prepared for nutrition 

 into the general mass of the juices, do not directly pertain to the 

 department of physiological chemistry, we must not overlook the 

 fact, that on the one hand, no definite limits can as yet be drawn 

 between the actions of affinity and purely mechanical molecular 

 motions, and on the other, that a scientific comprehension of the 

 whole process from a purely chemical point of view would be 

 impossible. If we here recur to the previous comparison between 

 the absorbing organs of the intestinal tract and the roots of the 

 higher plants, it at once follows that it is only a system of organs for 

 resorption that can be compared with the roots of plants, and that 

 even here the similarity is less between their mechanical configu- 

 ration than between the laws according to which the absorption 

 proceeds. These resorbing organs are the minute capillaries which 



