LUNGS. 



tnat notliing can pafs from one to the otlitr, h rendered 

 obvious ; lit, by anatomical examinations, in which they may 

 be feparated without any injury ; 2dly, by experiments on 

 dead bodies, in which fluids may be thrown into one pleura 

 without palling into the other; and jdly, by obfervations on 

 difcafcd fubjefts, in which water, pus, &c. are often con- 

 tained for long periods in one plei;ra only. 



In order to underftand the relations of the pleura to the 

 lungs and to the other thoracic organs, let us defcribe it as 

 if it began behind the fternum. From this part it extends 

 outwards, covering the ribs, their cartilages, and the internal 

 intercoftal mufc'es, confequcntly lining the fides of the cheft. 

 At the heads of the ribs it covers the ganglia of the great 

 fympathetic nerve, and their branches. When it has reached 

 the back of the cavity, and the vertebral column, indead of 

 pafling in front of that column, -it is continued from be- 

 hind forwards, on the fide of the aorta and the ccfophagus, 

 In front of which it is applied againll the membrane of 

 the oppofite fide, to form the potlerior part of the medi- 

 aftinum. It would be continued in this way from the fpine 

 to the fternum through the whole length of the chelt, if it 

 did not meet with the fafciculus of velTels entering the root 

 of the lung ; it is reflefled over thefe, and over ihe ferface 

 of the lung, to the fubftance of which it is clofely con- 

 nefted, forming its exterior covering. At the front of the 

 root of ths lung it covers the anterior furface of thefe 

 vetfels : th;n is continued from behind forwards on the fide 

 of the pericardium. It is then applied to the bag of the 

 oppofite fide, to form the front of the medialliiuim, and, 

 laftly, terminates on the back of the fternum, where we 

 fuppofed it to begin. Below it extends over the whole fu- 

 perior furface of the diaphragm. 



Thus it appears that the figure of the pleurae is com- 

 pletely determined by that of the cavity which the ti-.em- 

 branes line. Each of thefe bags is conical : it rifes into 

 an obtufe point within the fpace included by the firft rib ; 

 on the anterior, outer, and pofterior afpefts it is convex, 

 where it lines the ribs and intercoftal mufcles ; below it is 

 concave and expanded over the diaphragm ; and on the in- 

 fide, where it contributes to the mediaftir.um, nearly plane, 

 but (lightly concave in the fituation of the heart. Some 

 anatomifts diftinguifli in the pleura three portions, according 

 to the parts of the cheft wiiich they cover : -viz. the coftal, 

 the diaphragmatic, and the mediaftinal. 



The pleurae adhere with different degrees of firmnefs to 

 the parts which they line. The medium of this connection 

 is a cellular tifluej continuous below with tJiat of the ab- 

 dominal parieles, alcove with that of the neck and upper 

 extremities, and in all directions with that which fills the in- 

 terftices of the muLles forming the fides of the cavity. 

 In the neighbourhood of the vertebra:, and in fome parts of 

 the mediaftinum, this tifTue is copious, and often contains 

 fat : the adhefion is elofer to the ribs and intercollal muf- 

 fcles, and moft firm to the diaphragm. 



The relative fituation of the two bags varies at different 

 parts of the cheft, as different organs are interpofe.l be- 

 tween them. Towards the upper and anterior part, imme- 

 diately under their obtufe points, behind the arteries coming 

 from the arch of the aorta, and above the pericardium, 

 they are contiguous and feparated only hy cellular texture. 

 In the middle and lower part of the chell, they are widely 

 feparated by the intervention of the heart, pericardium, 

 large blood-veffels, &c. Hence the axis of the cavity muft 

 pals from above obliquely downwards and outwards. 



The ri^ht and left do not precifely correfpond to each 

 other. The former, on account of the oblique pofition of 

 the heart, lines nearly the whole pofterior furface of the 



fternum : hence it is broader than tlie left ; but it is at the 

 fame time ftiorter, becaufe the diaphragm is more ftrongly 

 arched on this fide. The left is applied, for the fpace of 

 fome inches, to the aorta. 



The capacity of the two pleura: taken together is about 

 one hundred cubic inches in the dead body. It is generally 

 larger in the male than in the female fcx ; and is very dif- 

 proportionately fmall before birth. The right exceeds the 

 left in the fame proportion as the right is larger than the 

 left lung. 



The meillajllnum is the partition which feparates the two 

 bags of the pleurx, and divides the cheft into a right and a 

 left fide. It is formed by the appofition of the two mem- 

 branous facs, and extends from the vertebral column to ihe 

 fternum and cartilages of the left ribs. We defcribe in it 

 tv,o lateral furfaces, a pofterior and an aniericr edge, a 

 bafis and an apex. The lateral furfaces are fmooth, and 

 contiguous to the internal furfaces of the lungs, except 

 where tiie pulmonary velTels enter thofe organs : they form 

 the inner portions of the two bags of the pleun. The 

 pofterior edge is attached to the fpine, of which it exadUy 

 follows the dircftion. The anterior is fixed in an oblique 

 line to the fternum above, and to its edge and the cartilages 

 of the left ribs below. Hence, if we thruft 5^ pointed in- 

 ftrument through the middle of that bone, it will penetrate 

 the right pleura, and not touch *he mediaflinum. The ob- 

 lique pofition of tlie heart fecms to carry with it, as a con- 

 fequence, this obliquity of the mediaftinrm. Yet this 

 difpofition does not hold univcrfally : in fome fubjects the 

 mediaftinum defcends along the middle of the fternum : in 

 others, which indeed are very rare, it is inclined from left 

 to right, fo that the right fide of the cheft is narrower 

 than the left. Sometimes the right layer of the mediaftinum 

 is fixed to the middle of the fternum, while the left is at- 

 tached at the articulations of the cartilages. The bafis of 

 the mediaftinum correfponds to the fuperior furface of the 

 diaphragm, and prefents a wide feparation of the pleura:, 

 lodging the heart and pericardium. The apex correfponds 

 to the upper end of the cheft ; it enclofes the trachea, 

 the ccfophagus, and the veffels and nerves which are entering 

 into or going out of the cheft. 



The .Tiediaftinura is formed by the two pleurae, which, in- 

 ftead of coming into contaft with each other, leave a con- 

 fiderable interval between them filled by various organs. 

 Above and in front, they lodge the thym.us ; below and in 

 front, the heart v.ith its pericardium, and the largs veffels 

 conne&ed to its bafis ; behind, the ccfophagus and aorta. 

 The two laniinE of the mediaftinum touch each otlier only 

 in front of the pericardium, between the lower end of the 

 thymus and the diaphragm, and behind that membrane, in 

 front of the cefophagus, from the firft dorfal vertebra to 

 the cardiac orifice of tlie, diaphragm. The latter circura- 

 ftance has occafioned a divifion of the mediaftinum into atj 

 anterior and a pofterior part ; the firft includes all that is 

 placed in front of the ccfoph.^gus, the latter all behind it. 

 Thefe divifions are often called anterior and pofterior, or 

 fternal and dorfal mediaftina. The former is the broadcil 

 and ftiorteft of the two ; it ends about the fifth or fixth 

 rib ; while the latter extends to the eleventh. 



In.the anterior mediaftinum, or triangular fpace placed 

 behind the fternum and cartilages of the left ribs, befides 

 fome fat and cellular fubftance, and fome abforbing glands, 

 we have the thymus, the trunks of the internal mammary 

 arteries, and the heart. In the pofterior mediaftinum, or 

 interval of the pleurs immediately in front of the verte- 

 bra:, are found, in addition to fome adipcus and cellular 

 texture, aj'.d feverai abforbing glands, tlie end of the trachea 



■with 



