LUNG S. 



-t'.-ith the commencement of the bronchi, the greatcft part 

 of the cefopliagus, with the nerves of the eighth pair, the 

 pulmonary ariery and veins, the defcending aorta, the tho- 

 racic dufl, and the vena azygos. 



When the flernum is rsiftl, in order to expofe the con- 

 ' tents of the thorax, the fpace feparating the two plonrs be- 

 .hind tliat bone, and forming the anterior mcdiaftitiuiti, is 

 increafed, becaiife the membranes are partly detached from 

 the tlernum and ribs, to which they before adhered. Tlie 

 cellular fubllance occupyinji; the interval becomes tilled with 

 •air, and is confequenily rendered more fcnfible. 



The lamina; compoling the mediatlinum are rather tliinner 

 than the pleurx in other fituations. They are united to 

 each other, and to the parts included between them, by a 

 cellular tilTue continuous with that of the general external 

 furface of thefe bags. This tidue, as well as that con- 

 nefting the different parts together, is tolerably copious, 

 -and contains more or lefs fat. It is fufceptible of iiifiam- 

 malion and fuppuratio", as in other parts of the body ; but 

 •lefs frequently. Hence abfccffes fometimes occur here. The 

 laminae of the mediaftinum arc, however, more clc-fely at- 

 tached to the furface of the pericardium ; it is diflicu't 

 to feparate the two membranes completely. The right 

 lamina is more tenfe than the left ; a conliderable protu- 

 berance is obferved in the latter, and formed by the fitua- 

 tion of the heart : hence the former only, if either, can have 

 any effeil in fupporting the diaphragm. 



The mediadinum receives its peculiar blood-veffels. In 

 front its arteries come trom the internal mammary, and 

 the comes nervi phrenici ; behind, from the inferior thy- 

 roid, the fiiperior intercoltal, the pericardiac, ocfophageal, 

 and bronchial arteries. The veins correfpond to, and accom- 

 pany thefe. 



The media;1inum divides the thcil into two lateral halves; 

 feparatcs the membranous bags containing the lungs, and 

 renders their aftion independent of each other. It con- 

 fines alfo to one fide various morbid afFcclions, as effu- 

 ilons of blood, pus, Sec. Some phyfiologills conceive that 

 it is further ulctul, by fupporting the weight of the oppo- 

 fite lung, when we lie on the one fide, and protecting there- 

 fore the lung of the fame fide on which we lie. 



The pleura is nearly tranfparent, fo that we can eafily 

 diilinguifh through it the colours of the fubjacent parts. 

 The fibres of the diaphragm and intercollal mufcles, the 

 intercoilal veffels and the ribs are immediately difcerncd 

 through this membrane. We can (till more clearly perceive 

 all the fhades of colour in the lung through the pleura 

 puhnonalis ; this indeed is thinner than the pleura coftalis, 

 and adheres very clolely to the organ. Boili::g dellroys this 

 tranfparency, and gives the membrane a dirtyifii white ap- 

 ^pearance. It is very ItroTig in proportion to its ihicknefs. 

 Concerning its inimate organization we have nothing far- 

 ther to fay than what the reader will find in the general ac- 

 count of the ferous membranes. 



The arteries and veins of the pleura, befides what have 

 been already meotioned as belonging to the mediaftinum, 

 are derived chiefly from the interco tal vefFels. The ab- 

 fjrbents are exceedingly numerous, and pafs through glands 

 fituated about the heads of the ribs to the thoracic diict. 



Th- internal furface of the pleura is in all parts fmooth, 

 pale, and covered by a ferous moilfi'.re produced from the 

 cxhahng veffels of the membrane. This obfervation applies 

 as well to the pleura pulmonahs, as to the p. coftalis : the 

 jn'crnal furface of the former conilitutes the outer furface of 

 .the lung : the two portions of the membrane are contiguous 

 atall points. In a living animal, or in one recently fiaugh- 

 tered, and opened whilit yet warm, this ferous exhalation cf- 



capes in the form of a light whitifh fmokc : the furface of 

 the membrane has a foft llippery feel, but no aftual fluid is 

 difcerncd in the cheft. When the body has cooled, this 

 vapour is condcnfed into a few drops of liquor, which is 

 foon increafed by the tranfudaiion through the b'ood-veflels, 

 and then it conilitutes what authors have defcribed under the 

 name of liquor pleura;. It has been queflioned whether this, 

 or the fluid of the pericardium, which is alfo in very fmall 

 quantity, compoied the watery part of what iffued from the 

 lide of our Savinur when pierced by tlie foldicr. We are of 

 opinion, that the period, at which this occurred after death, 

 was too recent for us to fuppofe that any fluid, or if any, 

 not more than an exceedingly minute portion of fluid, could 

 be contained in the pleura or pericardium. Confcquentl) 

 the fac\ does not admit of a natural explanation, but mud 

 be referred to the miraculous powers fo fignally exerted in 

 other refpefts on this occafion. 



This ferous exhalation is conftantly abforhcd and renewed. 

 It keeps the lung in an infulated (late, and feparatcs it from 

 the parietes of the thorax. How far this is eflential to the 

 funftions of the organ, will be examined prefently. 



The exhalation of the pleura is varioufly changed in difeafe. 

 It confills of aftual fluid, either dcpofited in unufual abun- 

 dance, or not abforbed with the ufual activity, in hydro-tho- 

 rax. In pleuritis it is the coagulating part of the blood, 

 which afterwards forms the adhelions ot the two pleurae, fo 

 commonly feer. in the dead body, that hardly any fubjett is 

 entirely free from them. 



As the pleura pulmonalis and coftalis are always conti- 

 guous, it follows that the lung always fills the cavity of this 

 membranous bag. If we diffeft away carefully the mufcular 

 ■ parts, that fill the interval of two ribs, fo as to expofe, 

 without penetrating the pleura, the tranfparency of the lat- 

 ter membrane allows us to fee the lung through it, and to fee 

 that there is no interval between them, but that they are in 

 accurate contaft in all parts of the cheft. The relult of this 

 examination is the fame, both in the living and the dead fub- 

 jeft. From this reprefentation it follows that the motions of 

 the cheft muft be accompanied by correfponding changes of 

 the lungs ; that air will enter into or pafs out of thofe organs 

 through the trachea, which is conftantly open, according as 

 the cheft is enlargt'd or diminiflied ; and, in faft, that the 

 dilatation and contraflion of the thorax are conftantly at- 

 tended with a fimilar dilatation and contraction of the lungs. 

 Thefe motions of the cheft refer entirely to the funftions of 

 the lungs, which are paffive in refpiration, which poflels in 

 themfelves no independent power of enlargement and dimi- 

 nution. 



Different opinions were, for a long time, entertained on 

 this fubjeCt ; it was funpofed that a fpace filled with air, fe- 

 parated the lungs from the containing cavity. A frequent 

 &nd careful performance of the difiection mentioned above, 

 has however (hewn the hmg always in contact with the 

 pleura, when the latter has not been injured ; and the cheft 

 has been opened under water without a (ingle particle of air 

 efcaping. ' Indeed it is only by this contact that the function 

 of refpiration can be explained, if we admit the pafTive na- 

 ture of the lung : the expanfion and contraction of the 

 clieft would be no longer attended with enlargement and di- 

 minution of the lung, if air were contained between this 

 vifcus and the fides of its containing cavity. 



All the preceding obfervatious apply to the natural ftate 

 of the parts, in which the bag of the pleura is entire ; if 

 that be wounded, fo as to make a communication between 

 its cavity and the external air, the lung no longer continues 

 in contact with the fides of the cheft. It has been almoft 

 univerfally received, that when an opening is made into the 



thorax 



