390 ON RESPIRATION. 



ably exemplified, in the provision which is made for it in every living being; 

 such provision being more universally found than that for any other function, 

 except for the ingestion of aliment, and for the perpetuation of the race. Even 

 in Plants, a true Respiration is continually going on, although its effects are 

 sometimes obscured by those of a converse change, which is subservient to a 

 different purpose. It has been ascertained, that the absorption of oxygen, and 

 the extrication of carbonic acid never cease during the life of the plant, 

 taking place under all conditions, by day and by night, in sunshine and in 

 shade. This is their true Respiration. But Plants obtain from the atmo- 

 sphere a large proportion of the carbon which they require as food ; and this 

 they procure, by decomposing the carbonic acid of the air, absorbing or fixing 

 its carbon, and setting free its oxygen. Now to this process, which is only 

 performed by the green parts of plants, and under the influence of light, the 

 name Digestion has not improperly been given. A healthy Plant will fix in 

 this manner much more carbon than it sets free by Respiration ; so that its 

 effect upon the atmosphere is, on the whole, to aid in purifying it from the 

 deleterious ingredient so largely imparted to it by Animal Respiration, Com- 

 bustion, &c. The Fungi, however, derive their support, like Animals, only 

 from matter which has been previously organized: and their respiration is 

 uncompensated by the fixation of carbon from the atmosphere. The same is 

 the case, during the processes of flowering and germination in the higher 

 Plants ; for certain chemical conversions are then taking place, which involve 

 the liberation of a large amount of carbonic acid, and a corresponding absorp- 

 tion of oxygen, without any counterbalancing change.* In no Plants is there 

 any distinct respiratory circulation ; since the nutritious fluid can be brought 

 into close relation with the air, in almost every part of its course. There is, 

 however, a rudiment of an internal respiratory apparatus, in a system of air- 

 vessels, or tracheae, composed of membranous tubes kept pervious by an 

 elastic spiral fibre which winds within them, and closely resembling the air- 

 tubes of Insects. 



523. In the Animal kingdom we almost universally find distinct organs 

 for the aeration of the blood ; these are always formed upon the same general 

 plan, being essentially composed of a membranous prolongation of the external 

 surface, adapted by its vascularity and permeability, to bring the blood into 

 close relation with the surrounding medium. But as this medium may be 

 either air or water, we find two principal forms of the apparatus ; one of them 

 adapted for each kind of respiration. In aquatic animals, the membrane is 

 usually prolonged externally into tufts or fringes, which are so arranged as to 

 expose the greatest amount of surface to the water ; each filament of which 

 these are composed includes an afferent and efferent capillary vessel ; and it 

 is whilst the fluid is passing through them, that its aeration is accomplished. 

 The collection of tufts or fringes constitutes what are known as gills; and 

 though their arrangement varies considerably, their essential character is 

 but little different throughout the classes of animals that possess them. On 

 the other hand, in air-breathing Animals, the aerating surface is reflected in- 

 wardly, forming passages or chambers, into which the air is received, and on 

 the walls of which the blood is distributed in a minute capillary network. 

 Such a conformation is found even among some of the lower Articulata, which 

 have a series of air-sacs disposed along each side of the body, one for every 

 segment. In Insects we find, instead of these sacs, a system of prolonged 

 tubes, ramifying through the body, and carrying air into its minutest portions. 

 Even in some Mollusca, such as the Snail and other terrestrial Gasteropods, 

 we find a provision for aerial respiration ; a large cavity being formed in the 



* See Principles of General and Comparative Physiology, 440 etseq. 



