Digestion is a compound function easily reducible to some of the simple ones which have been enumerated. It consists of the motion of the stomach and contiguous parts, of the secretion of a liquid from its internal surface, and of that heat, which is the common result of all action, whether mechanical, vital, or mental, and which is better explained by such motion, than by chymical theories. Similarly compound are respiration and generation.
Thus, there is no organ nor function which is not involved by the simple and natural arrangement here sketched.
Compound, however, as the organs of digestion, respiration, and generation, are, yet, as they form so important a part of the system, it may be asked, with which of these classes they are most allied. The answer is obvious. All of them consist of tubular vessels of various diameter; and all of them transmit and transmute liquids. Possessing such strong characteristics of the nutritive or vital system, they are evidently most allied to it.
In short, digestion prepares the nutritive or vital matter, which is taken up by absorption—the first of the simple nutritive functions; respiration renovates it in the very middle of its course—between the two portions of the simple function of circulation; and generation, dependant on secretion—the last of these functions, communicates this nutritive matter, or propagates vitality to a new series of beings. In such arrangement, the digestive organs, therefore, precede, and the generative follow, the simple nutritive organs; while the respiratory occupy a middle place between the venous and the arterial circulation.
Nothing can be more improper, as the preceding observations show, than considering any one of these as a distinct class.
More fully, therefore, to enumerate the nutritive or vital organs, we may say, that, under them, are classed, first, the organs of digestion, the external and internal absorbent surfaces, and the vessels which absorb from these surfaces, or the organs of absorption; second, the heart, lungs, and bloodvessels, which derive their contents (the blood) from the absorbed lymph, or the organs of circulation; and third, the secreting cavities, glands, &c., which separate various matters from the blood, or the organs of secretion, and of which generation is the sequel.
[24] Appendix E.
[25] In perfect consistency with the assertion, that, though the organs of digestion, respiration, and generation, were really compound, still they were chiefly nutritive or vital, and properly belonged to that class, it is not less remarkable, that, in this division of the body, they are found to occupy that part, the trunk, in which the chief simple nutritive organs are contained. This also shows the impropriety of reckoning any of these a separate system from the vital.
[26] The bones resemble these, in containing the greatest quantity of earthy mineral matter.
[27] It is the possession of vessels which constitutes the vitality of vegetables.