LUNGS. 



wliok> fubftance is found to h<^ compofcc! of tliofc cells, as \vA\ 

 as its external fiirfjce. Injedtion with qiiickfilver will do- 

 mon Urate the fame (Irudure. This gives to ihc lung, wlien 

 cut or torn, a porous aad Ipoiiirv appearance tliroiighoiit. 

 The cells, when attentively furveyed on the fiirface of tiie lung, 

 have a roundilh liguro, but their outline is often irregnhtr. 

 When inflated they meafure ilh or ,' th of a line in diani»t.r. 

 They communicate together in nil direiilions fo completely, 

 by the ramifications of the air-tubes, that air might pafs 

 eafdy from a lingle cell into all parts of the lung; but tlie 

 cells of the neighbouring lobules do not feera to haw any 

 diredl commutjication. 



The mucous membrane, in an extremely delicate ftate, 

 continued from the minute ends of the air-tubes, is fuppofed 

 to compofe thefe cells ; but the niinntenefs of the objects 

 renders our defcription of them, excepting a fe«' general 

 fads, rather uncertain. Anatomy difcovers to us rather a 

 fpongy net-work, filled with air, and formed by blood- 

 vefleis croffing in every direction, than any clear arrange- 

 ment of diltindl cells, connected to the bronchial ramifica- 

 tions, like grapes to their flalk, aa they have been defcribcd 

 and drawn by feveral anatomifts. 



In this view of the fubjeft, the extent of furfacc of the 

 mucous membrane muil be enormous. Many attempts have 

 been made to exprefs it in numbers. Hales makes tlie air- 

 cclls -j-^jidth of an inch in diameter; the lurface of the air- 

 tubes equal to 1033: fquare inchej ; and that of the air-cells 

 to 20,000. Keil ellimates the number of the veficles at 

 174,418,615, and the whole internal furface of the lung at 

 21,906 fquare inches. Lieberkulm carries his eliimate'of 

 the iurface as high as 1500 cubic feet. We mention thefe 

 circumftances only to fliew the great extent of the mucous 

 membrane, and nut bccaufe we place much faith in their ac- 

 curacy. In reading defcriptions of the minute flnifture of 

 the lungs, and, indeed, in all other analogous parts of ana- 

 tomy, we fhould always bear in mind the obfervation of 

 Haller ; " Ea fere hoimnum eft infclicitas, ut omnis ultima 

 rerum phyficarum hilloria parum fu-ma fit, et ut altera ilia, 

 rerum geftarum memoratrix, in mythices fines terminatur." 



A mucous fluid conftantly lubricates the whole of this 

 furface. It is limpid, mild, and nearly infipid, or (lightly 

 fakifli, aad but little tenacious in the natural ftate. When 

 free from air, it finks in water. It is produced in fo fmall a 

 quantity, that it feenis to be diffolved in the air, and thus 

 to pafb off iufeniibly in exfpiration, or to be taken up by the 

 abforbents. It is poured out much more abundantly under 

 various circumftances, and is altered m colour and confid- 

 ence : it is then expelled by the expiratory efforts which 

 conlHtute cough. In children it has a reddifli colour ; and 

 it is often rather livid in adults. 



The watery vapoqr difcharged from the lungs in exfpira- 

 tion concurs in lubricating the furface ol the air-paflages. 

 Whether there be any exhalation from the general mucous 

 furface, in addition to the mucous fccretiou, feeras a point 

 hsrdly fufceptible of pofitive determination. 



The pulmonary mucous membrane is the part in which 

 the chemical phenomena of refpiration are carried on ; its 

 furface is in contadl with the air taken into the lungs. The 

 latter fluid is the on-Iy one, in addition to its natural mucus, 

 of which it can bear the contact. All other fubftances, 

 even the clearelt water, are immediately and powerfully re- 

 jcfted by it. In itfelf it pcfTefles no power of expelhng any 

 irritating matter from its furface; but it excites the ex- 

 piratory nuifcles which adl convulfively and repeatedly until 

 ii\" caufe is removed. 



The velTels and nerves of the lungs are principally diftri- 



bated on thff air-tuber, of which t"hcy every whn-f foUovr 

 the couriv.-. The artiries of the trachea come from the in 

 ferior thyroidcal ; thofe of the bronchi tire derived immedi- 

 ately fronT the aorta, and are called the bronchial arteries. 

 The latter ramify in conjunftion with the air-tubes, and ad- 

 here cloiely to them : they are dillributed cliiefly on the 

 internal membrane. Several branches are fpent on the 

 bronchial glands, and on lli€ parietes of the pulmonary vef- 

 fels. D:i they anafiomofe with the pulnionarv artery ? We 

 ihould be inclined to fuppofe that they <!o «ot, from con- 

 fidering that the two kinds of vefl'els cf.nlain blood of dif. 

 ferent natures, and have altogether different offices. TJie 

 bronchial arteries convey arterial blood for the nutrition of" 

 the lung ; the pulmonary artery takes the venous blood, 

 that it may be fubmitted to the aftion of the air in rcfpira- 

 tion. Yet it has been afl'erted by feveral anatomifts, that 

 fuch an anallomofis does take place. The bronchial veins 

 accompany the arteries: their trunks end in the \':na 

 azygos on the right fide, and in the fuperiur intercuflal vein 

 on the left. 



The par vagum furnifhes nearly all the nerves belonging 

 to thefe organs : thofif of the trachea come from it alto- 

 gether. The nerves of the bronchi are derived from the 

 pulmonary plexufes formed principally by the par vagum, 

 but partly alio by branches from fome ganglia of the great 

 fympathctic. As thefe nerves feem to be dillributed entirely 

 on the air-tubes, perhaps the epithet bronchia would be 

 more proper for them than pulmonary. 



TIk' vafcular fyilem of the lungs may be divided into 

 three parts, dillinct in their nature, properties, and the 

 immediate ohjeif of their phenomena. The firll is the pul- 

 monary artery, or the end of the general fyflem of black, 

 blood ; the fecond, the pulmonary veins, or commencement 

 of the general fyllem of red blood; and tlie third, the ca- 

 pillary iyflem intermediate to the two preceding. (See 

 Circulation- and He.\rt.) The pulmonary artery arifes 

 from the upper and anterior part of the right ventricle, in 

 front of the origin of the aorta. It paffes upwards and 

 backwards, lying clofe on the left fide of the root of the 

 aorta, and, after a courfe of about an inch and a half, di. 

 vides into a right and left branch det'uned for the corre- 

 fponding lungs. Thefe two divifions feparate moil widely 

 at their origin, going off from the trunk at right angles, 

 and hardly forming a fenfible angle with each other. The 

 feparation takes place on the left fide of the aorta. The 

 right trunk goes immediately behind the aorta and luperior. 

 vena cava, and follows a tranfverfe courfe to the right lung ; 

 the left has an analogous courfe en the left fide of the body, 

 but is much fiiorter than the right, on account of the latter 

 paliQng beiiind the aorta and vena cava. Both arc placed in 

 front of the bronchi, and crofs the courfe at firll, being di- 

 reeled a little from below upwards ; but they are fubdivided 

 exaftly like the bronchi, and follow their courfe, being 

 clofely cojineded to them throughout ; the artery is gene- 

 rally placed above its correfponding air-tube. For the or» 

 ganifation of this vcffel, we refer to tlic article Heaht. 



The pulmonary veins, arifir.g from the capillary fyftem, 

 follow a courfe analogous to that of the arterial divifions. 

 Thefe alio accompany the air-tubes, and are fituated under 

 them. They unite gradually into larger and larger tubes, 

 and form ultimately four confiderable trunks,;tvvo belonging 

 to each lung, and terminating in the left auricle of the heart. 

 The fuperior right pulmonary vein paffes out of the lung 

 below tlw bronchus, and goes with a little obliquity down. ■ 

 wards : tlie inferior afcends obliquely towards the auricle. 

 The left veins have a fimilar arrangement; one defccnds, 

 aiid the other ^fcenda: they are more approximated than oa^ 

 IS t^ 



