86 THE HUMAN LUNGS. 



pleura, including under that title all the cellular tissue which is 

 directly continuous with the pleura. This pleura is a skin envelop- 

 ing each lung; the cellular tissue is a web of skins that dips down 

 into the substance of the lungs, and separates stem from stem, 

 bough from bough, branch from branch, and twig from twig ; at 

 once dividing the parts from each other, and uniting them into a 

 common body. The cellular tissue is therefore the bed in which 

 the several parts of the lungs are planted. As it runs between the 

 parts, and makes them into aggregate portions, or lobes and little 

 lobes, it is sometimes called the interlobular tissue. 



The lungs which we have thus endeavored to construct, are two 

 conical organs, filling, with the heart, the cavity of the chest. They 

 correspond in shape with the inside of the chest, and press below 

 upon the diaphragm. The pleura which covers them, contracts and 

 dilates with every respiration, and maintains its spring during life. 

 The elasticity of this serous membrane is an indication that where 

 serous membranes are present, as for instance over the brain, and 

 over the abdominal viscera, a similar elasticity is intended, or an ex- 

 pansile and contractile motion like breathing is performed. 



The chest, in which the lungs are placed, is a conical box, move- 

 able in its parts, and capable not only of dilatation and contraction, 

 but of infinite variations of shape, ylt harmonizes with the lungs 

 in their movements, forming with them but one machine, so that it' 

 is indifferent whether we say that we breathe with the lungs or with 

 the chest. In good health, when consent between the two is per- 

 fect, the bones and muscles of the ribs are of no heaviness in func- 

 tion, but rock and swim upon the lungs. Thus when we speak of 

 the lungs in the sequel, we imply the whole engine of breathing 

 even to the skin, and regard the chest itself as a dress or membrane 

 inseparable from the lung-principle. 



Inspiration, or the drawing in of the breath, is caused by certain 

 muscles drawing out the walls of the chest, and enlarging its inward 

 cavity, in which case the pressure of the external column of atmos- 

 phere causes the air to rush down into the windpipe and fill the 

 lungs, which then enlarge to fill the cavity of the chest. Expira- 

 tion or breathing out depends upon the relaxation of the muscles 

 and the resiliency of the parts of the chest, as well as upon the elasti- 



