RESPIRATORY SYSTEM. 229 



perceived to issue from the interior of" the cavity. This vapour, 

 shortly after death, becomes condensed and converted into a 

 hquid ; which accounts for the contiguous surfaces of the pleura 

 being moist, and for a collection of more or less fluid, resembling- 

 water, existing in the most depending parts of the cavity. In 

 consequence of every part of the membrane being bedewed in 

 this manner, the lung itself may be said to be in an insulated 

 state ; for the pleura costalis does not, philosophically speaking, 

 touch the pleura pulmonalis, nor is the latter in actual contact 

 with the mediastinum : all friction therefore, in the motions of 

 these parts, is by this interfluent secretion effectually prevented. 

 In this, then, consists the chief use of the pleura, viz. to furnish 

 a secretion for the purposes of lubrication and facility of motion, 

 which it further promotes by its extreme glibness of surface. It 

 is said also to answer the purpose of ligameiiis to the contained 

 organs, thereby confining and strengthening them. The use of 

 the mediastinum is to divide the chest into two compartments. 



Lungs. 



The lungs (by butchers called the lights) are two spongy bodies 

 formed for the purpose of respiration. 



Situation and Relation. — They are contained in the lateral 

 regions or sides of the thoracic cavity ; separated from each other 

 by the mediastinum and heart, which occupy the middle region. 

 Prior to any opening being made into the thorax, the lungs con- 

 tinue to fill up every vacuity : no sooner, however, is a perforation 

 made into the thoracic cavity than they shrink in volume, and be- 

 come in appearance too small for the spaces they occupy. This 

 arises from their being during life — or rather during the un- 

 opened state of the thorax — in a constant state of inflation with 

 atmospheric air, which preserves them expanded ; and they suffer 

 collapse of substance the instant air is admitted, in consequence 

 of the pressure of the atmosphere upon them, from which they 

 were protected before by the parietes of the thorax. 



Division. — The lungs are two in number : the right and the 

 left lung: partitioned from each other by the mediastinum. A 

 further division of these organs has been made into lobes: — that 

 on the right side, the larger of the two, consists of three lobes ; 

 the left, only of two : these lobes, which are nothing more than 

 partial divisions of the lung by fissures of variable extent through 

 its substance, serve to adapt them more accurately to the thoracic 

 cavities, and, at the same time, render them fitter for the pur- 

 poses of expansion and contraction. 



