EXCRETION. 



153 



Dr. Prout has lately stated strong reasons for 

 thinking, that great part of the contents of the 

 lymphatic vessels are not excrementitious, but 

 destined for useful purposes in the animal eco- 

 nomy; remarking particularly on the way in 

 which hybernating animals appear to be nou- 

 rished by absorption of their own fat* 



And it is obviously possible, that the excre- 

 tions may be required to purify the blood of 

 matters taken in from without, or evolved in 

 the course of the circulation and its abundant 

 changes, as well as to purify it of what has 

 been absorbed from the system itself. Now 

 that we know, that great part of the ingesta into 

 the stomach are taken up by the veins, and pass 

 through the liver on their way to the heart ; and, 

 likewise, that the venous blood is the chief 

 source of the excretions of bile, it seems pro- 

 bable, that one important use of this excretion 

 is, to subject a part of the ingesta to a second 

 filtration, or rejection of part of their ingre- 

 dients, subsidiary to that which they undergo in 

 the primae viae. This may also be probably one 

 principal reason why the great mass of the chyle, 

 and other products of absorption in the body, 

 should be mixed with the blood just before its 

 concentration at the heart, and subsequent dif- 

 fusion through the lungs ; and thus participate 

 in a purification, by the rejection of water and 

 carbonic acid, before they are applied to the 

 purposes of nutrition. We know, that in birds, 

 reptiles, and fishes, there is a venous circulation 

 similar to that of the vena portac, through the 

 substance of the kidneys, of most of the blood 

 coming from the lower half of the body ; a 

 part of the ingredients of that blood will, there- 

 fore, be evolved with the urine; and, in the 

 case of the reptiles, it has been lately ascertained, 

 that this venous blood receives, before entering 

 the kidneys, the contents of numerous and large 

 lymphatics .-)- 



At all events, if we are right in supposing, 

 that, in the higher animals, all the great chemi- 

 cal changes which are wrought on the blood, 

 even the formation of the excretions, are effected 

 during its circulation in the bloodvessels them- 

 selves, we can thereby acquire a general notion 

 of the intention of several contrivances, the use 

 of which is otherwise very obscure. We can 

 understand, that the object of the concentration 

 of the blood at the heart may be not merely 

 mechanical, but, partly, also chemical ; and we 

 can see the intention of the heart being so ad- 

 mirably adapted, by the articulated structure of 

 its internal surfaces, not only to receive and 

 propel, but also most effectually to intermix, all 

 the component particles of the blood, both be- 

 fore and after its exposure to the air ; the most 

 perfect illustration of which power of the heart 

 is afforded by the effect it produces on any com- 

 pressible and elastic fluid which is received in 

 a mass of any considerable volume into its cavi- 

 ties, and which is necessarily subdivided into so 

 many minute globules, and compressed in so 

 many directions, that it cannot escape from the 

 heart, and.so stops the circulation. 



* Bridgewater Treatise, p. 515, et seq. 

 t M tiller, in Phil. Transactions, 1833. 



Again, when we attend to the manner in 

 which substances foreign to the circulation are 

 absorbed into it, whether from the system itself, 

 or from without, we see a great deal of contri- 

 vance, evidently adapted, and probably intended, 

 to secure the raosi gradual introduction, and the 

 most perfect intermixture possible, and to allow 

 the escape of certain parts of the compound 

 fluid formed. Thus of the contents of the 

 primae vise, part are absorbed into the veins, and 

 sent through the capillaries of the liver and 

 those of the lungs, (both admitting of excretion,) 

 before they are admitted into the arteries. 

 What is taken up by the lacteals has already 

 undergone much elaboration by living fluids ; 

 this portion passes through the mesenteric glands, 

 and is, probably, so far intermixed with the 

 blood there, and partly received into the veins 

 passing from them to the liver;* and the rest is 

 mixed with much matter flowing from other 

 parts of the system by the lymphatics; and, 

 according to the views of Dr. Proutf as to the 

 nature of absorption, is so far assimilated by 

 this mixture also, before it is poured into the 

 great veins in the state of chyle, to undergo the 

 thorough agitation at both sides of the heart, 

 and to participate in the changes at the lungs. 

 What is absorbed from other parts of the body 

 seems to be partly taken up by the veins, partly 

 also by lymphatics which immediately convey 

 it into adjacent veins; the remainder passes 

 through lymphatic glands, and is there pretty 

 certainly subjected to an intermixture and an 

 interchange of particles with blood; after 

 which it has necessarily much further admix- 

 ture, and two thorough agitations at the heart, 

 as well as the exposure at the lungs, to undergo, 

 before arriving at the left side of the heart. 



In those of the vertebrated animals which 

 have no lymphatic glands, the thorough inter- 

 mixture of the fluids contained in the lymphatic 

 vessels is provided for by numerous plexuses,! 

 and, in the case of reptiles, by distinct lympha- 

 tic hearts communicating with veins ; and we 

 are sure, that much of the matters absorbed in 

 these animals, whether by veins or lymphatics, 

 passes through the capillaries of the kidneys or 

 liver, as well as the lungs, before reaching the 

 arteries. 



When we see so much contrivance, evidently 

 adapted for giving every facility to the gradual 

 operation of the vital affinities subsisting among 

 the constituents of the blood, before it reaches 

 the scene of any of the acts of nutrition, secre- 

 tion, or excretion, we cannot be surprised to 

 find, that these acts themselves should appear 

 to be so simple as the observations already 

 quoted would seem to indicate. 



It must be admitted, that if we consider these 

 contrivances in the higher animals as important 

 agents in the elaboration of the blood, and con- 

 sequent formation of the textures and prepared 

 fluids of the body, there is a difficulty in under- 

 standing how these objects can be accomplished 



* Tiedemann et Gmelin, Recherches, &c. 

 f Bfidgewater Treatise, ubi supra. 

 | Cuvier, Lemons, &c. t. iv. p. 98. 

 $ M tiller, ubi supra. 



