THEIR MODE OF ACTION. 393 



If it is assumed that chemical action and the vital principle 

 mutually balance each other in the blood, it must further be sup- 

 posed that the chemical powers will have a certain degree 

 of preponderance in the lungs, where the air and blood are in 

 immediate contact ; for these organs are fitted by nature to favor 

 chemical action ; they do not offer resistance to the changes 

 experienced by the venous blood. 



The contact of air with venous blood is limited to a very short 

 period of time by the motion of the heart, and any change beyond 

 a determinate point is, in a certain degree, prevented by the 

 rapid removal of the blood which has become arterialized. Any 

 disturbance in the functions of the heart, and any chemical 

 action from without, even though weak, occasions a change in 

 the process of respiration. Solid substances, also, such as dust 

 from vegetable, animal, or inorganic bodies, act in the same way 

 as they do in a saturated solution of a salt in the act of crystal- 

 lization, that is, they occasion a deposition of solid matters from 

 the blood, by which the action of the air upon the latter is altered 

 or prevented. 



When gaseous and decomposing substances, or those which 

 exercise a chemical action, such as sulphuretted hydrogen and 

 carbonic acid, obtain access to the lungs, they meet with less 

 resistance in this organ than in any other. The chemical pro- 

 cess of slow combustion in the lungs is accelerated by all sub- 

 stances in a state of decay or putrefaction, by ammonia and alka- 

 lies ; but is retarded by empyreumatic substances, volatile oils, 

 and acids. Sulphuretted hydrogen produces immediate decom- 

 position of the blood, and sulphurous acid combines with the 

 substance of the tissues, the cells, and membranes. 



When the process of respiration is modified by contact with a 

 matter in the progress of decay, when this matter communicates 

 the state of decomposition, of which it is the subject, to the blood, 

 disease is produced. 



If the matter undergoing decomposition is the product of a dis- 

 ease, it is called contagion ; but if it is the product of the decay 

 or putrefaction of animal and vegetable substances, or if it acts 

 by its chemical properties (not by the state in which it is), and 



