SOURCES OF CARBONIC ACID IN THE ANIMAL BODY. 361 



accordance with the external tenperature ; increasing with its diminu- 

 tion, as more heat must then be generated; and diminishing with its 

 increase. In all cases, if a sufficient supply of food be not furnished, 

 the store of fat is drawn upon ; and if this be exhausted, the animal 

 dies of cold ( 117). 



648. To recapitulate, then ; the sources of Carbonic acid in the Ani- 

 mal body are threefold. 1. The continual decay of the tissues ; which 

 is common to all organized bodies ; which is diminished by cold and 

 dryness, and increased by warmth and moisture ; which takes place 

 with increased rapidity at the approach of death, whether this affect 

 the body at large, or only an individual part ; and which goes on un- 

 checked, when the actions of nutrition have ceased altogether. 2. The 

 metamorphosis, which is peculiar to the Nervous and Muscular tissues ; 

 which is the very condition of their activity ; and which, therefore, 

 bears a direct relation to the degree in which they are exerted. 3. 

 The direct conversion of the carbon of the food into carbonic acid ; 

 which is peculiar to warm-blooded animals ; and which seems to vary 

 in quantity, in accordance with the amount of heat to be generated. 



649. Now the function of Respiration has for its object, not merely 

 to extricate the Carbonic acid which is generated in the system, but 

 likewise to introduce the Oxygen which is required to form that car- 

 bonic acid ; the proportion of oxygen in the tissues, and in the combus- 

 tible materials of the blood, not being sufficient for this purpose. 

 Hence it is not enough, that the carbonic acid should be removed ; for 

 this may be accomplished by causing an animal to breathe an atmo- 

 sphere which contains no oxygen. Any cold blooded animal, such as a 

 Frog or a Snail, may be kept in hydrogen or nitrogen for several hours 

 or even days ; and will give out, during that time, an amount of car- 

 bonic acid nearly as great, as if it had been respiring atmospheric air. 

 But the continued production of carbonic acid must have a limit, occa- 

 sioned by the want of oxygen, and death will then supervene. On 

 the other hand, a supply of oxygen may be freely afforded ; and yet 

 the presence of even a small amount of carbonic acid in the surround- 

 ing atmosphere (in addition to that which is normally present in it, 

 81) will impede the extrication of that substance from the blood ; 

 and if the excess be considerable, the carbonic acid will not be set free 

 at all ; so that the same injurious results follow, as if respiration were 

 altogether prevented from taking place. 



650. These two actions are accomplished by the very same act ; ad- 

 vantage being taken of the property of "mutual diffusion," which is 

 common to all gaseous substances that do not unite chemically with one 

 another. In virtue of this property, Hydrogen, the lightest of gases, 

 and Carbonic acid, one of the heaviest, when introduced into the same 

 vessel, will be found in a short time to have uniformly mixed, notwith- 

 standing the difference of their specific gravities, which are as 1 to 22. 

 Now this intermixture will take place, when the two gases are sepa- 

 rated by a porous septum ; each gas passing towards the other, by an 

 action resembling the Endosrnose and Exosmose of liquids ( 491). 

 And it may also take place, when one of the gases is diffused through 

 a liquid; provided that the other gas is likewise capable of being 



