LUNGS. 



the lungs. We have to confider it under this double point of 

 ■view. 



The refiftance of the cheft to the aftion of external force 

 IS dificrent on its anterior, poftcrior, and lateral afpefts. 

 I. The thick mufclcs placed behind annihilate the efkdls of 

 falls and blows on that part. Two prominences formed by 

 thefe mufcles, and feparated by a groove correfponding to 

 the fpinous proccffes, occupy the pl;:ce of tlie two longitu- 

 dinal chaimels obfcrvcd in the fl^elcton : thcfe fupport the 

 cffedts of blows. The provilions for protection in this htu- 

 ation, refer to the fpinal marrow as nuich as to the peftoral 

 viicera. 2. In front, where the mufcles are fewer, the 

 modeof relillance variosin infpiration ;ind exfpiration. When 

 the chell is ftrongly dilated, the llcrnum fupports any 

 effort directed againll it in the manner of an arcli, and more 

 tfiicacioully in proportion as the infpiration is ifronger. In 

 this way. individuals lying on tlieir backs lupport enorrrous 

 ■weights on the front of the chell : e g.s blackfmith's anvil, on 

 ■which a horfe-flioehrisat the fame time bei-n hammered. Here, 

 however, the mechanifm is not the fame as that by which the 

 cranium tupports a wt:ie;ht bearing on it perpendicularly : in 

 that cafe the bony arch alone is concerned, the mufcles have 

 nothing to do with the phenomenon. In the cheft, on the 

 contrary, the e.xternal mufcles, particularly the lerratus an- 

 ticus, draw the ribs llrongly outward":, and relift their de- 

 prcflton : they arc the active fupports of the bony arch re- 

 prefented by the cheft. If the force be fuperior to the re- 

 fiftance, fratlure cnfucs : this may either be dire£t, and 

 affett the flernum, which is rare ; or it may take place by 

 contrecoiip at the part which has experienced the greatelt 

 effort, as in the middle of the ribs, Thefe obfervations nia- 

 nifelUy applv to the true ribs only. Fraftnres by contre- 

 coup may alfo take place from fudden blows, where the 

 mufcles have no time to contradt and fupport the ribs, and 

 the latter are confeqnently left to the full operation of the 

 force. In exfpiration the cheft eludes the force by yielding : 

 the ribs are prefied inwards, and the vifcera are in lome de- 

 gree comprefted. This yielding ia much more manifcit in 

 the lower ribs. 3. The cfFe6ts of blows, or violence of 

 other kindb, affcdting the fides of the cheft, are obviated in 

 nearly the fame manner, whether in infpiration or exfpira- 

 tion. The arch rcprefented by the rib has its points of 

 fupport in the llcrnum and the vertebral column : the bone 

 may be direftly fraftured at the point where the violence is 

 oifered, which is the moft common cafe, or by coritrecoup, 

 which is more rare. The ftrength of the ligaments at the 

 anterior and pofterior- articulations is fo confiderable, and 

 the fupport adorded by the tranfverfe" proccfies fo firm, 

 that luxations cannot take place while the joints are in a 

 healthy ftate. The laft ribs, which terminate loofely in the 

 abdominal parietes, cannot be faid to offer any reiiftance to 

 the force which is applied to them : they yield towards the 

 abdomen. The firft and fecond ribs are fo covered by 

 external parts, that they can hardly be afFefted by blows or 

 other kinds of force. 



The motions of the cheft are direfted to two principal 

 objefts ; its dilatation and contraction, which correfpond to 

 infpiration and exfpiration. It may be dilated in tliree diiferent 

 directions ; perpendicularly, tranlverfely, and from before 

 backw,;rds. i. The diaphragm alone is the agent of per- 

 pendicular dilatation, and produces alone in the adult thofe 

 flight infpirations, in which but little air enters the cheft. Its 

 fleihy iides, which correfpond to the lungs, defcend much 

 more than the tendinous centre, which fupports the heart. 

 (See Diaphragm.) 2. In the next or greater degree of in- 

 spiration the cheft is firft dilated perpendicularly, and then 

 tranfverfely by the iatercoilal mufcles. (See Intekcos- 



TAi.r.s. ) Befides the caufes mentioned in that article for the 

 little power of motion in the firft rib, we may notice the 

 inconllderahle length and great breadth of its cartilage, and its 

 confolidation with the fternum, inftcad of bein;^ joined by 

 a moveable articuhition. The remarkable ftiortiiefsandbreadtlj 

 of the rib itfelf alfo concur m producing the fame cfleft. 

 In conicquence of the oblique pofition of the ribs, they 

 cannot be elevated without having their middle carried out- 

 wards, which produces a tranlverfe enlargement of the 

 cheft ; moreover, this elevation twifts the cartilages, which 

 throws the ribs ilill more outwards. 3. In the preceding 

 motion the ribs are carried a little forwards, and as this effedl 

 takes place in a greater degree below, where the ribs are 

 longeft, the fternum is carried forwards at its lower extre- 

 mity, tlie upper remaining nearly motiotdefs ; and the 

 cheft is confequently enlarged from before backwards. This 

 motion, however, is very Imali in an.ount, as we may afcertain 

 by obferving the refpiration of a lean individual ; it is 

 infignificant in companion to tlie motion by which the ribs 

 are carried outwards. As the llerHum correfponds to the 

 heart, while the ribs furround the lungs, erilar'.rement is 

 lefs needed in the former than the latter diredlion. 



Thecontraflion of the cheft, correfponding to exfpiration, 

 is effected by a mechanifm exaftly oppollte to that which 

 we have jull explained. It takes place from below upwards 

 by the elevation of the diaphragm. In the tranfverfe 

 direftion it is effeftcd by the denreffion of the ribs, which 

 are carried inwards by the twilled cartilages recovering their 

 original ftate. The elevation of the bone in infpiration pro- 

 duces a tvtifting of the cartilage ; and in exfpiration the re- 

 covery of the latter depreffes the former; fo that the bone 

 and the cartilage reciprocally determine motion in each other. 

 The effedt of this twifting of the cartilages muft not, how- 

 ever, be over-rated : in order to make it contiderable, they 

 ought to be confolidated to the ilernuin, whereas their arti- 

 culation to that bone allows a certain degree of motion. 

 The greater this motion, the lefs will be the tvi illlng ; and it 

 would not cxift at all if the articulation were loofe enough 

 to allow full fcope to the elevation of the rib. It cannot 

 have any effeft in the falfe ribs. In proportion as the ribs 

 defcend and are carried inwards, the fteniuin is alfo reftored, 

 its inferior portion pafTing backwards. 



All thefe movements, whether of dilatation or contrac- 

 tion, are much more fenfible at th.e lower part of the cheft, in 

 confcquence of the iilore extenllve motion enjoyed in this 

 part ; a circumftance that coincides with the greater breadth 

 of the inferior portion of the lungs. 



We have to point out, in the next place, the powers by 

 which thefe motions of the cheft areeffedtcd. We may dif- 

 tinguifh two kinds of changes taking place in the cheft ; an 

 enlargement and fubfeqiient contra'^tion in the perpendicular 

 direction, and another acting circularly. The diaphragm 

 is the iole agent of the perpendicular enlargement ; and as 

 it extrends the cheft downwards, where the cavity is moll 

 arnple, it produces a very confiderable dilatation. (See 

 Di.'\PllRAGM.) This niufcie can defcend three inches, or 

 more, and has four or five times as much effect in the en- 

 largement of the cheft, as all the other powers put together. 

 Hence injuries or difeafes of it produce the greateft diftur- 

 bance in the fundtion of reipiration. The perpendicular 

 contradlion is cfFedtcd by the abdominal mulcles ; that is, 

 by the obliqui externi and interni abdominis, the tranfverfi 

 and redli. Thefe, which form the fides and front of the 

 abdominal parietes, yield to the vifcera thruft downwards by 

 the defcent of the diaphragm : hence an elevation of the 

 belly is perceptible on infpiration. They then contradl, pufti 

 backwards and upwards the parts which had before defcendcd, 

 6 rellore 



