116 ANATOMY AND PHYSIOLOGY. 



The lungs, then, are contained in the chest or thorax, a conical 

 cavity formed by the sternum or breast-bone before, the back-bone 

 behind, and the ribs above and on the sides. It is separated in- 

 feriorly from the abdomen or belly by a fleshy movable partition 

 called the diaphragm, which is fixed to the bottom of the breast- 

 bone and edges of the short ribs before, and extends downwards 

 and backwards to be attached also to the spine behind. Through 

 this the cesophagus or gullet, blood-vessels, &c., pass. The whole 

 inside of the chest is lined by a thin smooth membrane called the 

 pleura, which divides the chest into a right and a left side, and 

 which likewise covers the lungs ; but these are, nevertheless, on 

 the outside of the pleura, in the same way as the head is on the 

 outside of the double nightcap. There is no opening to admit the 

 air between the lungs and sides of the chest, but it gets in easily 

 by the windpipe into the air-tubes of the lungs. 



From these explanations it will be easy to understand the 

 mechanism of respiration (breathing). Drawing in a breath is 

 called inspiration. We do this, 1st, by raising the ribs, which are 

 provided with numerous muscles for this purpose between the ribs, 

 and attached to the ribs and neck ; and, 2dly, and at the same 

 time, by depressing the diaphragm. Of the latter movement we 

 become sensible, by placing the hand on the abdomen during in- 

 spiration, when we notice the ribs raised, and find the belly pushed 

 outwards at the same moment by the descent of the diaphragm. It 

 is, therefore, evident that the cavity of the chest must be consider- 

 ably enlarged by inspiration. But the cavity of the chest cannot be 

 enlarged without something filling it up ; and as no air can get 

 between the lungs and sides of the chest, if the windpipe remain 

 open, the air necessarily rushes by it into the air-tubes and vesicles 

 of the lungs, and blows them up as we might blow up a 

 bladder. The expulsion of the air from the lungs is effected prin- 

 cipally by the elasticity of the ribs, and by the contraction of the 

 muscles of the belly pushing up the diaphragm. It is called expi- 

 ration. 



It must be manifest, from considering these arrangements, that 

 the amount of blood and air brought together in the lungs must be 

 very great. The whole extent of the air-tubes, in man, taken col- 

 lectively, has been calculated by Hales at about twenty thousand 

 square inches, and by Monro at twenty times the surface of the 

 human body ; the branches of the pulmonary artery, which ramify 

 upon this surface, are so twined and interlaced, that they have re- 





