570 OF RESPIRATION. 



CHAPTER XIII. 



OF RESPIRATION. 



1. Nature of the Function: and Provisions for its Performance. 



749. IT is obvious that the Nutritive fluid, in its circulation through the 

 capillaries of the system, must undergo great alterations, both in its physical 

 constitution, and its vital properties. It gives up to the tissues with which it 

 is brought into contact, some of its most important elements ; and, at the same 

 time, it is made the vehicle of the removal, from these tissues, of ingredients 

 which are no longer in the state of combination, that fits them for their offices 

 in the Animal Economy. To separate these ingredients from the general cur- 

 rent of the circulation, and to carry them out of the system, is the great object 

 of the Excretory organs ; and it is very evident that the importance of the 

 respective functions of these will vary with the amount of the ingredient 

 which they have to separate, and with the deleterious influence which its re- 

 tention would exert on the welfare of the system at large. Of all these injuri- 

 ous ingredients, Carbonic Acid is without doubt the one most abundantly 

 introduced into the nutritive fluid; and it is also most deleterious in its effects 

 on the system, if allowed to accumulate. We find, accordingly, that the pro- 

 vision for the removal of Carbonic Acid from the Blood, is one of peculiar 

 extent and importance, especially in the higher forms of Animals ; and further, 

 that instead of being effected by an operation peculiarly vital (like other acts 

 of Excretion), its performance is secured by being made to depend upon simple 

 physical laws, and is not nearly so susceptible of derangement from disorder 

 of other processes, as it would be if its conditions were less simple. All that 

 is requisite for it, as we shall presently see, is the exposure of the Blood to 

 the influence of the Atmospheric air, or of Air dissolved in water, through the 

 medium of a membrane that shall permit the diffusion of gases; and an inter- 

 change then takes place between the gaseous matters on the two sides, Car- 

 bonic acid being exhaled from the Blood, and being replaced by Oxygen. 

 Thus the extrication of Carbonic acid is effected in a manner, that renders it 

 subservient to the introduction of the element which is required for all the 

 most active manifestations of vital power; and it is in these two processes 

 conjointly, not in either alone, that the function of Respiration essentially con- 

 sists. We shall now inquire into the sources from which Carbonic acid is 

 produced in the living body; and the causes of the demand for Oxygen. 



7f)0. All organized bodies, as already explained, are liable to continual 

 dec-ay, even whilst they are most actively engaged in performing the actions 

 of Life ; and one of the chief products of that decay is Carbonic Acid. A 

 large quantity of this gas is set free, during the decomposition of almost every 

 kind of organized matter; the Carbon of the substance being united with the 

 oxygen supplied by the air. Hence we find, that the formation and liberation 

 of carbonic Acid go on with great rapidity after death, both in the Plant and 

 in the Animal ; and that they take plaae, also, to a very great extent, in the 

 period that often precedes the death oFtlie body, during which a general de- 

 composition of the tissues is occurring. Thus in Plants, as soon as they 

 become unhealthy, the extrication of carbon in the form of carbonic acid takes 



