142 ANIMAL. 



our survey of the structure we have already effecting immediately the composition and de- 

 seen to how great an extent the organization composition of the parts and particles of the 

 became complicated as a consequence of this organization. It is intermediate to the pre- 

 centralization of the office of digestion, and paration of the nutritious juices and their ap- 

 with what variety of superadded function this propriation or assimilation by the organism. 

 complication was attended, namely, external The lacteals or absorbent vessels of the in- 

 absorption, sanguification or the formation of testines collect the fluid called chyle, from the 

 a fluid, the pabulum of nutrition, confined pultaceous alimentary mass in its progress 

 within vessels, respiration, circulation, and, through the intestines. But this fluid is not 

 finally, assimilation, in regard to the compo- yet fitted to subserve nutrition; as a pre- 

 sition ; whilst with reference to the vital de- liminary it has to be subjected to the action 

 compositions we have discovered another spe- of the atmospheric air in the gills, lungs, &c., 

 cies of interstitial or internal absorption, and where, being converted into arterial blood, it 

 depuration of the system by one principal first becomes apt to minister to the growth 

 apparatus, the kidney, to which the cutaneous and reparation of the body and its parts. So 

 and pulmonary exhalations may be added as also in regard to decomposition : the fluids 

 supplementary. collected from all parts by the lymphatics and 



But every one of these functions, and its veins, are not immediately rejected from the 



organic apparatus, are themselves modified, economy, as useless and having already accom- 



according to internal aptitude, and in con- plished all of which they are susceptible, but 



formity with the circumstances surrounded by being first exposed to the contact of the at- 



which animals commence and continue their mosphere, and then made to undergo the 



existence. Digestion is a very simple process scrutiny of the depurative organs, they are 



in those cases in which it takes place within either retained, being restored to their pristine 



a single cavity, having but one opening, and capacity to subserve nutrition, or are abstracted 



no complementary apparatus of any kind, from and thrown out of the body as no longer 



compared with what it is when connected with fit to aid in its growth and maintenance, 

 an apparatus for bruising the food, for mixing Intercourse with the air of the atmosphere is 



it with saliva, for macerating it in a crop or a essential to every living thing, and we should a 



series of reticulated and fbliaceous pouches, priori have anticipated very considerable variety 



mixing it with bile, pancreatic juice, &cc. &c., in the means by which, as well as the mode in 



and transmitting it along a muscular canal, which this intercourse is established. Among 



of six, eight, or ten times the length of the the inferior tribes which are nourished by ab- 



body to which it belongs. sorption immediately from the surface of their 



Absorption, in like manner, among the most body, and which find the materials of their 

 inferior classes is essentially one and undi- nutrition ready prepared for their use in the 

 vided either in kind or destination. It is in circumambient media, we may presume that 

 itself adequate to the entire office of nutrition, the matters absorbed have either undergone 

 seizing and transmitting the matters which are the needful changes by exposure to the air 

 fitted for this end, elaborating the food and previously to their assumption, or that these 

 atmospheric air at the same instant of time, changes take place at the time they are ap- 

 and effecting immediately the composition of propriated. Where digestion is a preliminary 

 the whole animal organism. In animals higher to absorption and assimilation, it is evident 

 in the scale, we perceive, in the first place, that that this could not have been the case ; and 

 there are several species of absorption : there hence the necessity for that modification of the 

 is, in the first place, the absorption from the function of aeration en titled respiration. Look- 

 surface of the digestive passages and that from ing generally, we observe two principal varieties 

 the surface of the lungs, gills, skin, &c. or of in the mode by which aeration is accomplished: 

 the respiratory apparatus. Again, absorption is in some classes there are a number of holes 

 not limited to furnishing materials for the com- arranged symmetrically along the sides, and 

 position of the organism ; it is also entrusted communicating with air-vessels entitled tra- 

 with the office of abstracting from its interior chese, which are subsequently distributed to 

 the particles which are worn out and no longer every part of the body. The air in this case 

 fit to continue the ends of their existence in is evidently brought into communication with 

 the places they occupy. Nor is this all ; for the nutrient juices already arrived at their 

 it is by absorption that the amount of those destinations ; and the necessary changes are 

 exhaled fluids which moisten internal cavities, wrought in them at the instant of their assimi- 

 having no external communications, is regu- lation. Here the respiration is very properly 

 lated, and by which, as it would appear, many said to be diffuse, or disseminated. In other 

 of the secreted fluids, the bile, and the sper- classes, again, in which the respiration is local 

 matic fluid in particular, are inspissated and or concentrated, in harmony with the existence 

 rendered more fit to accomplish the important of a special apparatus, which we have spoken 

 ends they subserve in the economy. Absorp- of under the title of lung or gill, aeration is 

 tion in the highest classes of all is even per- accomplished by the access of the air on the 

 formed by two, and perhaps three different one hand, and the exposure to its action of the 

 orders of vessels, the lacteals, namely, the nutritive fluid on the other, the effect of which 

 lymphatics, and the veins. is to convert the latter into arterial blood, and 



Further, absorption is not in the higher as to make it fit, upon its distribution by appro- 

 it is in the lower classes of animals a function priate channels, to accomplish the ultimate and 



