S4 THE VITAL FUNCTIONS. 



amount of power necessary for its conversion into that pro- 

 duct. It is obvious, for example, that the chemical changes 

 which vciictable food must be made to under2;o, in order to 

 assimilate it to blood, must be considerably greater than 

 tiiuse recjuircd to convert animal food into the same fluid, 

 because the latter is itself derived, with only slight modifi- 

 cation, immediately from the blood. We accordingly find 

 it to be an established rule, that the digestive organs of ani- 

 mals which feed on vegetable materials are remarkable for 

 their size, their length, and their complication, when com- 

 pared witli those of carnivorous animals of the same class. 

 This rule applies, indeed, universally to Mammalia, Birds, 

 Reptiles, Fishes, and also to Insects: and below these we 

 can scarcely draw the comparison, because nearly all the 

 inferior tribes subsist wholly upon animal substances. Many 

 of these latter animals have organs capable of extracting 

 nourishment from substances which we should hardly ima- 

 gine contained any sensible portion. Thus, on examining 

 the stomach of the earth-worm, we find it always filled with 

 moist earth, which is devoured in large quantities, for the 

 sake of the minute portion of vegetable and animal materi- 

 als that happen to be intermixed with the soil; and this slen- 

 der nutriment is sufllcient for the subsistence of that ani- 

 mal. Many marine worms, in like manner, feed apparently 

 upon sand alone; but that sand is generally intermixed with 

 fragments of shells, which have been pulverized by the con- 

 tinual rolling of the tide and the surge; and the animal mat- 

 ter contained in these fragments, aflbrds them a supply of 

 nutriment adequate to their wants. It is evident, that when, 

 as in the preceding instances, large quantities of indigestible 

 materials are taken in along with such as are nutritious, the 

 stomach and other digestive cavities must be rendered more 

 than usually capacious. It is obvious also that the structure 

 of the digestive organs must bear a relation to the mechani- 

 cal texture, as well as the chemical qualities of the food; 

 and this we lind to be the case in a variety of instances, 

 which will hereafter be specified. 



