The first step by which nutrition becomes more complex, as we rise in • the scale of cre ation, is the institution of a process of solution (digestion), by which the matters appropriated as aliment are prepared for reception into the body. This process of solution is accom plished by powers inherent in the animal itself, within a cavity destined for the purpose. In our survey of the structure we have already seen to how great an extent the organization became complicated as a consequence of this centralization of the office of digestion, and with what variety of superadded function this complication was attended, namely, external absorption, sanguification or the formation of a fluid, the pabulum of nutrition, confined within vessels, respiration, circulation, and, finally, assimilation, in regard to the compo sition ; whilst with reference to the vital de compositions we have discovered another spe cies of interstitial or internal absorption, and depuration of the system by one principal apparatus, the kidney, to which the cutaneous and pulmonary exhalations may be added as supplementary.
But every one of these functions, and its organic apparatus, are themselves modified, according to internal aptitude, and in con formity with the circumstances surrounded by which animals commence and continue their existence. Digestion is a very simple process in those cases in which it takes place within a single cavity, having but one opening, and no complementary apparatus of any kind, compared with what it is when connected with an apparatus for bruising the food, for mixing it with saliva, for macerating it in a crop or a series of reticulated and foliaceous pouches, mixing it with bile, pancreatic juice, &c. &c., and transmitting it along a muscular canal, of six, eight, or ten times the length of the body to which it belongs.
Absorption, in like manner, among the most inferior classes is essentially one and undi vided either in kind or destination. It is in itself adequate to the entire office of nutrition, seizing and transmitting the matters which are fitted for this end, elaborating the food and atmospheric air at the same instant of time, and effecting immediately the composition of the whole animal organism. In animals higher in the scale, we perceive, in the first place, that there are several species of absorption : there is, in the first place, the absorption from the surface of the digestive passages and that from the surface of the lungs, gills, skin, &c. or of the respiratory apparatus. Again, absorption is not limited to furnishing materials for the com position of the organism ; it is also entrusted with the office of abstracting from its interior the particles which are worn out and no longer fit to continue the ends of their existence in the places they occupy. Nor is this all ; for it is by absorption that the amount of those exhaled fluids which moisten internal cavities, having no external communications, is regu lated, and by which, as it would appear, many of the secreted fluids, the bile, and the sper matic fluid in particular, are inspissated and rendered more fit to accomplish the important ends they subserve in the economy. Absorp tion in the highest classes of all is even per formed by two, and perhaps three different orders of vessels, the lacteals, namely, the lymphatics, and the veins.
Further, absorption is not in the higher as it is in the lower classes of animals a function effecting immediately the composition and de composition of the parts and particles of the organization. It is intermediate to the pre paration of the nutritious juices and their ap propriation or assimilation by the organism. The lacteals or absorbent vessels of the in testines collect the fluid called chyle from the pultaceous alimentary mass in its progress through the intestines. But this fluid is not yet fitted to subserve nutrition ; as a pre liminary it has to be subjected to the action of the atmospheric air in the gills, lungs, &c., where, being converted into arterial blood, it first becomes apt to minister to the growth and reparation of the body and its parts. So also in regard to decomposition : the fluids collected from all parts by the lymphatics and veins, are not immediately rejected from the economy, as useless and having already accom plished all of which they are susceptible, but being first exposed to the contact of the at mosphere, and then made to undergo the scrutiny of the depurative organs, they are either retained, being restored to their pristine capacity to subserve nutrition, or are abstracted from and thrown out of the body as no longer fit to aid in its growth and maintenance.
Intercourse with the air of the atmosphere is essential to every living thing, and we should a priori have anticipated very considerable variety in the means by which, as well as the mode in which this intercourse is established. Among the inferior tribes which are nourished by ab sorption immediately from the surface of their body, and which find the materials of their nutrition ready prepared for their use in the circumambient media, we may presume that the matters absorbed have either undergone the needful changes by exposure to the air previously to their assumption, or that these changes take place at the time they are ap propriated. Where digestion is a preliminary to absorption and assimilation, it is evident that this could not have been the case; and hence the necessity for that modification of the function of aeration entitled respiration. Look ing generally, we observe two principal varieties in the mode by which aeration is accomplished: in some classes there are a number of holes arranged symmetrically along the sides, and communicating with air-vessels entitled tra cheae, which are subsequently distributed to every part of the body. The air in this case is evidently brought into communication with the nutrient juices already arrived at their destinations ; and the necessary changes are wrought in them at the instant of their assimi lation. Here the respiration is very properly said to be diffuse or disseminated. In other classes, again, in which the respiration is local or concentrated, in harmony with the existence of a special apparatus, which we have spoken of under the title of lung or gill, aeration is accomplished by the access of the air on the one hand, and the exposure to its action of the nutritive fluid on the other, the effect of which is to convert the latter into arterial blood, and to make it fit, upon its distribution by appro priate channels, to accomplish the ultimate and immediate nourishment of every part of the organization.