232 RESPIRATION. 



CHAPTEE XII. 



BESPIRATION. 



THE blood as it circulates in the arterial system has a bright 

 scarlet color ; but as it passes through the capillaries it gradually 

 becomes darker, and on its arrival in the veins its color is a deep 

 purple, and in some parts of the body nearly black. There are, 

 therefore, two kinds of blood in the body ; arterial blood, which is 

 of a bright color, and venous blood, which is dark. Now it is found 

 that the dark-colored venous blood, which has been contaminated 

 by passing through the capillaries, is unfit for further circulation. 

 It is incapable, in this state, of supplying the organs with their 

 healthy stimulus and nutrition, and has become, on the contrary, 

 deleterious and poisonous. It is accordingly carried back to the 

 heart by the veins, and thence sent to the lungs, where it is recon- 

 verted into arterial blood. The process by which the venous blood 

 is thus arterialized and renovated, is known as the process of 

 respiration. 



This process takes place very actively in the higher animals, and 

 probably does so to a greater or less extent in all animals without 

 exception. Its essential conditions are that the circulating fluid 

 should be exposed to the influence of atmospheric air, or of an 

 aerated fluid ; that is, of a fluid holding atmospheric air or oxygen 

 in solution. The respiratory apparatus consists essentially of a 

 moist and permeable animal membrane, the respiratory membrane, 

 with the bloodvessels on one side of it, and the air or aerated fluid 

 on the other. The blood and the air, consequently, do not come in 

 direct contact with each other, but absorption and exhalation take 

 place from one to the other through the thin membrane which lies 

 between. 



The special anatomical arrangement of the respiratory apparatus 

 differs in different species of animals. In most of those inhabiting 

 the water, the respiratory organs have the form of gills or branchiae ; 

 that is, delicate filamentous prolongations of some part of the 



