118 SEROUS SECRETIONS. 



most wholly of water. The principal one of them is the 

 matter of perspiration secreted from the skin, and though 

 almost entirely composed of water, its expulsion from 

 the system is absolutely necessary to a state of good 

 health. Its quantity is so small that it is always imper- 

 ceptible to the eye, unless it be increased by an unusual 

 warmth of the body. Then it collects in large drops on 

 the skin, and is popularly called sweat. It has been as- 

 certained from experiment, that the whole quantity of 

 insensible perspiration, including that from the lungs, va- 

 ries from eleven to thirty-two grains in a minute. 



Emily. I never knew before, that the lungs perspi- 

 red ; pray how is that fact ascertained ? 



Dr. B. You have only to look at a person in a cold 

 day, when he is pouring out from mouth and nose suc- 

 cessive jets of warm vapour, which is condensed by the 

 cold air, to satisfy yourself of the fact ; or to save the 

 trouble of going out on purpose, just breathe a few times 

 on the mirror, and see the matter of perspiration con- 

 densed on its cold surface. 



Emily. I have observed all this, but I never imagined 

 it came from the lungs. 



Dr. B. A curious fact in the animal economy is, 

 the nice sympathy which exists between the lungs and 

 the skin. Hence, in a climate where the temperature 

 is constantly undergoing great and sudden changes, you 

 see a fruitful source of disease. The perspiration of the 

 skin is frequently checked the irritation is propagated 

 by sympathy to the lungs these delicate organs are 

 stimulated beyond their natural measure by the increased 

 action to which they are thus subjected -frequent repi- 

 tition of this undue excitement leads to inflammation and 

 change of structure and consumption is the common 

 termination of the scene. 



The serous secretions do not differ materially from 

 the serum of the blood, and are poured out over the 

 whole surface of the serous membranes. These mem- 

 branes line the cavities of the body which have no com- 



