374 POISONS, CONTAGIONS, MIASMS. 



easily as in the lungs, and it is well known that 

 diseases of the lungs are above all others frequent 

 and dangerous. 



If it is assumed that chemical action and the 

 vital principle mutually balance each other in the 

 blood, it must further be supposed that the chemi- 

 cal powers will have a certain degree of preponde- 

 rance in the lungs, where the air and blood are in 

 immediate contact ; for these organs are fitted by 

 nature to favour chemical action ; they offer no 

 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 arterialised. 

 Any disturbance in the functions of the heart, and 

 any chemical action from without, even though 

 weak, occasions a change in the process of respi- 

 ration. Solid substances also, such as dust from 

 vegetable, (meal,) animal, (wool,) and inorganic 

 bodies, act in the same way as they do in a satu- 

 rated solution of a salt in the act of crystallisation, 

 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 sul- 

 phuretted hydrogen and carbonic acid, obtain access 

 to the lungs, they meet with less resistance in this 



