PREDISPOSITION TO PULMONARY DISEASE. 171 



cells, and therefore of a magnitude proportioned to 

 the quantity of blood which is destined to pass 

 through them. 



These two tissues, air-tubes and blood-vessels, 

 together with the loose cellular texture or net- 

 work which binds them together, called paren- 

 chyma, form the whole of the structure peculiar to 

 the lungs. But, like all other living parts, they are 

 provided also with nerves, without the active co- 

 operation of which, in supplying the requisite ner 

 vous stimulus, the special functions of the lungs, and 

 consequently life itself, would speedily cease. 



Every one must have remarked the copious exha- 

 lation of moisture which takes place in breathing, 

 and which presents a striking analogy to the exha- 

 lation from the surface of the skin. In the former 

 as in the latter instance, the exhalation is carried on 

 by the innumerable minute capillary vessels, in 

 which the small arterial branches terminate in the 

 air-cells. This can be made evident after death, by 

 injecting any of the arterial branches with water, 

 turpentine, or quicksilver, when the injection will be 

 seen to exude in minute points, on the surface of 

 the lining membrane of the air-cells. The pulmo- 

 nary exhalation, however, must not thence be sup- 

 posed to be a mere physical or mechanical exuda- 

 tion. It is the result of a vital process, and is sub- 

 ject to the ordinary laws of vital action. 



Absorption^ in like manner, takes place from the 

 lining membrane of the lungs, as we have seen it do 

 n the skin. When a person breathes an atmo- 

 sphereloaded with fumes of spirits, of tobacco, of 

 turpentine, or of any other volatile substance, a 

 portion of the fumes is taken up by the absorbing 

 vessels of the lungs, and carried into the system, 

 and there produces precisely the same effects as if 

 introduced into the stomach. It has occasionally 

 happened that a person has unwarily become intox- 

 icated in this way; and the lungs thus become a 



