60 PHYSIOLOGY. 



into arterial ; carbonic acid, given off from, and oxygen taken into 

 the system. In the higher classes of animals the whole of the cir- 

 culating fluid is sent through special organs formed on the same 

 general principle as the secreting glands, which fulfil incessantly 

 an office the most essential to life ; these organs are the lungs. 

 In the lungs, the mass of the circulating fluid, which had been 

 changed in the periphery of the body into venous blood, mixed with 

 the lymph from all parts, and the newly-elaborated chyle, is brought 

 into intimate contact with the air of the atmosphere, the effect of 

 which is to restore to the blood its bright colour, and to give to it the 

 arterial character which is alone competent to minister to nutrition, 

 and to impart to the nervous system and locomotive apparatus their 

 proper stimulus. The arterial blood, thus changed, also supplies to 

 the secretory apparatus the material for the exercise of its function. 

 All that is necessary then for these purposes is, that the blood should 

 be exposed to the influence of the atmospheric air, or air dissolved 

 in water, through the medium of a membrane that shall permit the 

 diffusion of gases ; an interchange then takes place between the 

 gaseous matters on the two sides, — carbonic acid being exhaled from 

 the blood and replaced by oxygen. 



The extrication of carbonic acid is effected in a manner that renders 

 it subservient to the introduction of oxygen, which is required for all 

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

 actions conjointly, not in either alone, that the function of respiration 

 essentially consists. It will be remembered that the carbonic acid 

 passes out through the animal membrane by exosmose, and the oxygen 

 passes in by endosmose. 



The sources of the carbonic acid given off* in respiration are three- 

 fold, 1st. 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 affects the body at 

 laro^e, or only an individual part ; and which goes on unchecked 

 when the actions of nutrition have ceased altogether. 



2d. 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. 



3d. 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.* 



The organs of respiration are always formed upon the same 



* For a more detailed account of these sources of carbonic acid, of which the 

 above is merely a recapitulation, see Carpenter's Principles of Human Physiol- 

 ogy, 3d American Edition, p. 570 and seq. 



