CONCOCTION. [Lesson xxrvni. 



their blood-vessels : for we shall then clearly per- 

 ceive the change which their fabric and actjon. 

 must produce on the chyle. 



The windpipe is composed of segments of car- 

 tilaginous rings on the fore part, to giv.e a free 

 passage to the air in respiration j and of a strong 

 membrane on its back part, to bend with the 

 neck, and give way to the gullet in deglutition. 

 This pipe is lined throughout with an infinity of 

 glands, which perpetually distil an unctuous dense 

 humour to lubricate and anoint the passages of 

 the air. Soon after the windpipe has descendecl 

 into the cavity of the breast, it is divided into 

 two great branches ; and these two are subdivided 

 into innumerable ramifications called bronchia, 

 which become smaller in their progress (not much 

 unlike a bushy tree inverted), until at last they ter- 

 minate in millions of Jittle bladders which hang 

 in clusters at their extremities, and are inflated 

 by their admission of air, and subside at its ex- 

 pulsion. The plusters constitute the lobes of the 

 lungs, 



The blood-vessels of the lungs next deserve^ur 

 attention. The branches of the pulmonary artery 

 run along with those of the windpipe, and are ul- 

 timately subdivided into an endle.ss number of 

 capillary ramifications, which are spread, like a fine 

 net work, over the surface of every individual air- 

 bladder ; and the pulmonary vein, whose extreme 

 branches receive the blood and chyle from those of 

 the arteries, run likewise in form of a net over all 

 the air-bladders of the brpnchia. 



From 



