1 PHYSIOLOGY AND HEALTH. 



life, will bear the presence of pure air, but will not tolerate, 

 for a single moment, the presence of any other substance, 

 not even a drop of water, as most of us have had occasion to 

 know in some part of our lives. This lining is thin and 

 sensitive, and very liable to be disordered. Most of our 

 colds and catarrhs are but affections of this membrane. 

 And our coughs are mostly caused by some irritation applied 

 to it, or derangement in it. Whenever it is not in a com- 

 fortable condition, when it is dry for want of mucus, or 

 covered with too much of it, or when any particle of food, 

 or any other foreign substance, is lodged in any part of the 

 air passages or cells, or even gets within the glottis, then 

 nature sets up a violent expulsory effort to press the air out 

 suddenly, and to blow and force away the intruder, or relieve 

 the irritation. This is coughing. 



284. We perceive this sometimes when we breathe dust, 

 or offensive gases, or pungent matters, all of which irritate 

 and offend this sensitive texture. But, delicate as it is, it 

 may, by use, lose its sensibility, and become accustomed to 

 bear very injurious substances, as the sole of the barefoot 

 boy loses its delicacy, and will bear the rough surface of the 

 street without suffering ; and as the hands of the smith and 

 of the dyer lose much of their sensitiveness to heat, so that 

 they can, without apparent pain, handle iron and plunge into 

 dyes so hot as to burn others; so the membrane of the lungs 

 becomes used even to tobacco-smoke, and bears its frequent 

 and almost perpetual presence, without appearing to suffer 

 any harm. 



285. The heart is placed about the middle of the chest, 

 and between the lobes of the lungs, (205,) (Fig. XVIII. 

 p. 127,) and sends its blood-vessels to the right and to 

 the left, through each of these lobes. These vessels ramify 

 through every part of the organ, and are interwoven with the 

 air-tubes (Fig. XX.) These myriads of minute arteries come 

 in contact with the air-cells, and are separated from them 

 only by an exceedingly thin membrane, so thin that gasea 



