142 



ANIMAL. 



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 



