CHEMICAL PHENOMENA OF RESPIRATION. 323 



duce the reductions which we have mentioned, and again this 

 heat may be transformed into vis viva, when the combustion 

 of the vegetable tissues takes place. 



This is precisely the office performed by animals, whose 

 tissues consume the elements furnished by the vegetable 

 kingdom, oxidize and decompose them into carbonic acid and 

 water, thus producing heat and force (two synonymous or 

 equivalent words: see p. 78, mechanical equivalent of heat). 

 The interior phenomena of nutrition oxidize carbon, hydro- 

 gen, and sulphur : nitrogen apparently yields less readily to 

 this organic oxidation, and the urea, which represents the final 

 product of combustion of the albuminoids, contains nitrogen 

 which is in a free state, or at least not combined with oxygen, 

 because the urea is estimated by decomposing it into car- 

 bonic acid and nitrogen 1 (by means of Millon's reagent, 

 Grehant; see "Physiology of the kidney"). 



2. The Office of the Blood in Respiration. In those animals 

 which are ranked in a higher class than that of the articulata, 

 the blood serves as an intermedium between the tissues 

 and the respirable mediums. It cannot, however, be said that 

 the blood breathes for the tissues; it neither consumes 

 oxygen, nor produces carbonic acid, but is loaded with these 

 gases, simply for the purpose of famishing the tissues with 

 the former, and carrying the latter to those surfaces by which 



1 " By comparing the general laws of nutrition of vegetables 

 and animals, we find that the phenomena of nutrition are not alike 

 in the two kingdoms, but that they depend upon each other in 

 exact proportion to their dissimilarity." (Wundt, " Physiologic.") 

 The plant (the green parts) forms a combustible material which the 

 animal consumes: it operates by way of synthesis, being an appa- 

 ratus for reduction which rejects the oxygen. The animal takes 

 from the plant, either directly (as the herbivora) or indirectly (as 

 the carnivora), the carbonates and other substances, and consumes 

 these. The animal works by analysis, and is an apparatus for 

 oxidation. By means of this series of metamorphoses matter 

 passes from the inorganic to the 'vegetable kingdom, and thence 

 back into the animal kingdom, and again returns to the Inorganic 

 kingdom: earth and air, plant and animal, earth and air, forms an 

 unbroken chain: such is the rotation of matter. It must not, how- 

 ever, be supposed that there is no exception to this rule, for reductions 

 are sometimes observed in animal organisms, as well as oxidations 

 in vegetable organisms: neither is there any well-defined boundary 

 existing between the two kingdoms. (See, on this subject, "La 

 Circulation de la Vie," by J. Moleschott, French translation 

 Paris, 1866.) 



