LUNGS. 



eonfider them ns nuifcular, others rertard them as a fibrous 

 organ, to which their appearance is very fiinilar. 'J'liis 

 membrane alone conf 'tiites tlic folic! portion of the trachea 

 behind ; and hence arifes the flattened ligure ofthe tube 

 at that part. This peculiarity in the back of the trachea 

 has been referred to the fituntion and motions of the afo- 

 phagus, wliich lies clofe behind it : but the fame ftrufture 

 exilis in the bronchi alfo, where it cannot admit of that 

 explanation. The arrangement is different on the anterior 

 part and fides of the trachea, in two-thirds at leafl of the 

 circumference of the tube. The fibrous membrane is inter- 

 rupted by portions of cartilage (annuli cartilaginei), which 

 keep it on the llretch, and thereby preferve the air-lubes 

 conltantly open. Each of thefe cartilages reprcfents two- 

 thirds of atircle. Tiiey are bent on themfelves, flattened 

 on their furfaces, uniform in length, but of diflFercnt breadths. 

 Their convexity forms a part of the exterior furface of the 

 tub/ ; their concavity correfponds to the mucous mem- 

 branes, from which a thin celiidar ftratum feparates them. 

 Thi-ir fuporior and inferior margins are rounded, continuous 

 on the outfide with the fibrous tlffue, and (lightly prominent 

 through the mucous membrane on the infide. Their extre- 

 mities projeft more or lefs behind in the fibrous tidue, are 

 alien the fame level, are rounded, and a little bent upwards. 

 They vary in number from about iixtecn to twenty ; they 

 may be more or lefs broad ; and, as the length of the tube is 

 nearly uniform, they mull be more numerous in proportion 

 as they are narrower. In general, they are broader in front, 

 and diminidi progrelTively to the back part : but the reverfe 

 of this fometimes happens ; or two may be u^iited at their 

 edges. Commonly their direftion is liorizontal to the axis 

 of the trachea ; but many of them are often more or lefs 

 oblique. 



Id colour and texture they refcmble the fibro-cartiiages 

 of the organs of fenfe : they are very elaftic. 



The firll cartilage is generally much larger than the fuc- 

 ceeding ones ; the lall has a prolongation from its middle 

 correfponding to the bifurcation of the bronchi. 



In the ramifications of the bronchi the cartilages become 

 lefs regular in their form, and fewer. They no longer ex- 

 hibit that annular form, but coufiil merely of fmall pieces, 

 fometimes feparatc, and fometimes united. As the fubdivi- 

 fions are mukiphed, the cartilages become lefs firm, and at 

 laft difappear altogetlier, fo that we find only a membranous 

 flrufture when we have traced the air-tubes as far as the eye 

 can go. 



The external furface of this fibrous membrane is fprinkled 

 behind with fmall lightifh brown and flattened bodies of 

 very variable figure, round, oval, &c. Thefe are mucous 

 glands, of which the excretory dufts open on the internal 

 furface of the tube. Tliey are fmaller on the bronchi than 

 in the trunk of the trachea, and they become more minute 

 in proportion as the veffels ramify. Their (trufture feems 

 to be very fimple ; one duft comes from each gland geaerally ; 

 but fometimes two or three glands are united, and there 

 the dufts are more numerous. The fibrous membrane ex- 

 hibits none of thefe glands in the intervals of the cartilages, 

 on its external furface. The inner furface of the membrane, 

 in the fame fituation, correfponds to the mucous lining of 

 the tube, from which it is feparated by numerous fmall and 

 clofely arranged granular bodies, which are probably mucous 

 glands. At the back part of the tube there is found, under 

 the fibrous membrane, a (Ira'ium of tranfverfe fibres extended 

 between the extremities of the cartilages, to which they are 

 attached. Thefe are difpofed in fmall fafciculi, have not 

 the white aponeurotic appearance of the fibrous membrane, 

 a&d feem to be real mufcular iibrea. They are corinefted 



14 



to the raucous lining by a loofe cellular tilTue, and may be 

 molt advantas;eoufly feen by dilfefting a^ay that membrane 

 «froni the infide. In what manner ihefe fibres affect the 

 phenomena that occur in the trachea we do not know. 

 The longitudinal ones, that compofe the fibrous membrane 

 already defcribed, have generally been regarded as of a 

 mnfcular nature : but their appearance by no means warrants 

 thisreprefentation. Tiiey polTefsconfiderable elaflic power, fo 

 tliat the trachea, when extended, recovers itfe'f very quickly 

 and completely : this property is frequently brought into 

 exercife in the living (late from the motions of the head and 

 neck. The effetl of the cartilaginous femi-circles, wliich 

 are incorporated with this membrane, in preferving the air- 

 tubes permanently open, and the neceffity of this arrat;ge- 

 ment to thii execution of the refpiratory fundlions, arc too 

 obvious to require atiy detailed illullration. 



The mucous membrane, or the third conflituent portionr 

 of the air-tubes, is the fecond divilion of that great mucous- 

 apparatus, called by Bichat gajlro-pulmonaire. We trace its- 

 continuation from the pharynx into the larynx, through that 

 cavity into the trachea and bronchi, to their ultimate rami- 

 fications. Ill the latter, it is faid to cxill alone, or without 

 the other two parts already mentioned : but the minutenefs 

 of the parts makes it difTicult to afcertain this point by direft 

 examination, although fuch a flrufture would be favourable 

 to that more intimate conneftion between the blood and air- 

 vcffels, which is neceffary to the chemical phenomena of 

 icfpiration, according to flie notions commonly entertained 

 refpefting thefe phenomena. The external furface corre- 

 fponds behind to the tranfverfe fibres ; and in. the reft of its 

 extent to the fibrous membrane and cartilages, which latter 

 project through it. The connecting medium which attaches 

 it to all thefe parts is a cellular fubftancc, admitting eafily 

 of feparation. The internal furface is fmooth, 'and con- 

 ltantly lubricated by a mucous fecretion : it forms the 

 cavity of the air-tubes. The excretory canals of the mucous 

 glands open on it in many parts very dillindlly. In the 

 back of the tube, where there is no cartilage, it exhibits 

 numerous prominent and regular longitudinal folds : thefe 

 extend into the bronchi and their ramifications. They are 

 not produced by the contraftion of any parts fituated ex- 

 teriorly, but exift when the membrane has been detached^ 

 and are not affefted by tranfverfe extenfion of tlie tube : 

 they feem to arife from fmall fibrous columns form.ing a 

 part of the flrufture of the membrane. 



tor the organization of tliis membrane, we mud refer to 

 the general view exhibited in the article Membrane. It 

 is thinner, lefs fpongy and foft, and more firmly attached i:i 

 the trachea than in the larynx : the orifices of the mucous- 

 dufts are alfo fmaller ; in the bronchi it is Hill more delicate, 

 and this tiiinnefs increafes as the tubes rlivide. In the na- 

 tural flate it is white, fo as to indicate that the capillary 

 fylleni is not very clearly marked in it. Thefe veffels are 

 developed and become perceptible under numerous circum- 

 ftances, particularly in catarrhal affedtions, to which the 

 pulmonary mucous membrane is very fubjeft. ITic blood 

 is then accumulated in the capillaries, and gives to the mem- 

 brane a red colour, which it does not poffcfs naturally. 



It is fuppofed tliat this membrane compotes entirely the 

 air-cells, or veficle.s of the Uuigs, in which the minute rami- 

 fications of the air-tubts end. If we impel air mto the 

 trachea, the whole lung becomes diileuded, and increafes in 

 volume in proportion to the quantity of air inflated. At-> 

 tentive obfervation will then convince us that its whole 

 fubltance is compofed of fmall cells, which we can readily 

 difcern on the furface. If the inflated ung be dried, or i£ 

 the organ be diilendcd with ipirit of wine, and then cut, its 



whole- 



