DIGESTIVE CANAL (Comp. Anat.)--• The digestive canal is that cavity of the body which is destined to receive the food of animals and to retain it until its nutritious part has been separated or absorbed. It is termed also the alimentary or the intestinal canal. As it is the part into which foreign matter is first conveyed for the nutriment of system, its forms and structure are most intimately related to the kind of food, and consequently to the living habits and instincts, and the whole mechanism of animals. The most universal organs in the animal kingdom are the digestive, and most of the others may be considered as secondary or subservient to these. The lowest animals pre sent us with no other organs than those sub servient to digestion, and almost all the organs which are superadded to these as we ascend in the scale either form an extension of the nutri tive apparatus, or are destined to regulate the kind of food admitted into the alimentary cavity. An animal, in the abstract, may almost be viewed as a moving sac, organized to con vert foreign matter into its own likeness, and all the complex organs of animal life are but auxiliaries to this primitive digestive bag. The bones and other hard parts which form the solid frame-work of the body connected toge ther by their various ligaments serve only as firm levers to enable the active organs, the muscles, to carry it to and fro, and the ner vous system with its various organs of sense serve but to direct its motions in quest of food. Nature has placed the unorganized food of plants on the exterior of their body, and their vessels are sent there to seek it, which roots them through life to a fixed point; but animals place their food in their stomach and have their roots directed inwards and towards that central reservoir, so that they can move about and select what is most congenial to their nature. The organs of anitnol life relate to this diffe rence between the two organized kingdoms—. to this locomotion of animals and their power of selecting their food; but the organs of vege tative life of which the alimentary canal is the first, relate merely to the assimilation of food when already within the body, and are there fore common to animals with plants. The
digestive surface of the plant is the surface of its root, ramified and fixed in the soil, which affords it a never-failing supply of food; so that the vegetable is like an animal with its stomach turned inside out. The organs of relation are necessarily connected with the varied circumstances in which animals are placed, and are remarkable for their variable character, and even for their inconstancy in the lower tribes, where they are often entirely want ing; but those of vegetative or organic life are more regular and constant in their character, and indeed no organ is more universal among animals than that internal digestive cavity by which they differ so much from the species of the vegetable kingdom. This primitive sac is but a development or a continuation of the mucous surface of the skin, which extends into the homogeneous cellular tissue of the body, or completely through it; and although, in the simplest conditions of animals, it per forms alone all the assimilative functions, we find it, as we ascend in the scale, giving origin to various other systems to which distinct parts of the complex function of assimilation are entrusted. Thus the peripheral mode of nutri tion of the plant passes insensibly into the central internal mode of the animal, and all the organs of organic life, whether they open into the digestive cavity within, or on the surface of the body without, may be considered as originating from the skin, which is itself only a portion of the primitive cellular tissue of the body, here modified by the contact of the sur rounding element so as to assume the character of a mucous membrane. As the various tubular prolongations become more and more developed and isolated from this primitive source, they assume properties more and more peculiar, and thus form the numerous glandular appara tus and vascular systems.