LUNGS. 



^hat of the external furface, and is compofed of the fame 

 ■parts. The poilerior has in the middle a prominence formed 

 iy the bodies of the dorfal vertebrx, concave from above 

 downwards, and dividing the chefl into two internal halves. 

 On each fide of this is a confiderable elongated concavity, 

 narrow above, broad below, deeper in the middle than in any 

 other part, and containing the pollerior convexity of the lungs. 

 The lateral regions are concave ; farmed by the internal 

 furfaces of the ribs and the intercoftal fpaces. The pleura 

 lines them, as well as the pofterior hollows and the pec- 

 toral portion, excepting as much of the latter as corre- 

 fponds to the mediaftinum. 



The fuperior circumference is fmall, in coraparifon with 

 the inferior, and reprefents an oval placed tranfvcrfely. It 

 is formed behind by the vertebral column, in front by the 

 fternum, and on the fides by the firll ribs : the clavicles pro- 

 jeft a little at their inner ends, fo as to contraft this open- 

 ing in a flight degree. The trachea, the oefophagus, the 

 large blood-veffels, which either go from the heart to the 

 upper parts of the body, or return from the latter to the 

 Tieart, and feveral important nerves pafs through this open- 



The inferior circumference is very large, at lead four 

 times greater than the former, and differs from it in being 

 fufceplible of enlargement and contraftion. The fuperior, 

 •formed by two ribs nearly immoveable, preferves always the 

 fame capacity, and is, moreover, protefted by a conliderable 

 thicknefs of parts from the imprelHons of extraneous bodies, 

 that might tend to contraft it. To the mobility of the in- 

 ferior circumference are chiefly owing the variations in the 

 -.dimenfions of the cheft, produced by iiifpiration and exfpira- 

 tion, by caufes which aft on it from within outwards, and 

 dilate it, as dropfy, pregnancy, and the various abdo- 

 minal tumours, or by thofe which affect it in the contrary 

 ■way, and tend to contrail it, as the flays of women. It 

 ihould be obferved, at the fame time, that the vifcera placed 

 at this circumference can accommodate themfclves to thefe 

 varied dimenfions : while thofe which pafs through the fupe- 

 rior aperture, particularly the. trachea, would be affefted 

 very dangeroudy by any contraction of its fides. In this 

 inferior circumference there is a large notch in front, of a 

 triangular figure, with the bafis do^A'Uwards, the fides of 

 ■\vliich are formed by the edges of the cartilages of the falfe 

 ribs : in the apex of the triangle the enfiform cartilage pro- 

 j-cfs. On each fide of this notch there is a convex edge, 

 formed by the cartilages of the falfe ribs. Behind thefe 

 convexities there is a fmall notch on each fide, formed by 

 t!\e inclination of the laft rib, with refpeft to the vertebral 

 column. Several of the abdominal inulcles are attached 

 to all parts cf this circumference. 



Developement cf the Chejl. — The heart and the thymus, 

 which are fituatcd on the median li le of the chefl, in the 

 fcEtus, and are of very confiderable fize, require a propor- 

 tional extent in the antero-pofterior diameters, which then 

 predominate, while the tranfverfe are comparatively fmall, 

 on account of the imperfeft condition of the lungs. The 

 Uernum, feparated by a wide interval from the fpine, makes 

 3 confiderable prominence in front, fo that a large fpace is 

 left for the heart and thymus. The pollerior foffte, at the 

 fides of the vertebral column, are fmall, as the ribs are not 

 much curved at this part : hence the prominences behind, 

 formed at the fides of the fpine by the curves of the rib'3, 

 are not fenfible at this time. The cheft is particularly nar- 

 row in ihis direction at the angles of the ribs. The want 

 of this pollerior curvature is the principal caufe of the aug- 

 mentation of the antero-pofterior diameters. In fad, the 

 ribs are nearly as long, proportionally, at this time as after- 



wards; but they fwell lefs behind and at the fides, are 

 thrown more forwards, and, confcquently, carry the fternum 

 in that direftion. Thefe curves arc formed in the progrefs 

 of age ; the pofterior fofta: of the chefl are, confequently, 

 devt loped, and the fternum comes nearer to the fpine. The 

 tranfverfe diameters arc noiv increafed ; but the general 

 capacity of the cheft is not much augmented in proportion to 

 o'liier parts, as it lofes in one direttion what it gains in the 

 other, and its differences in the ftetus, and in the fubfequent 

 times, are referrible to the different relations of its dianrieters. 

 Thefe changes affedt the fuperior and inferior circuraferences. 

 The former is more capacious from before backv.'ards, but 

 lefs from fide to fide : the latter is very wide between the 

 enfiform cartilage and the fpine ; it is one-third wider here 

 than in the adult, in proportion. The tranfverfe diameteri 

 are lefs contradled here than in the reft; of the cheft ; fo that 

 the inferior circumference altogether is remarkable for its 

 great capacity in the fcetus, a difpofition which is accom- 

 modated to the very marked volume of the gaftric vifccra> 

 and particularly of the liver, which it includes. 



The different bones of the cheft are not developed in an 

 uniform proportion. The ribs are almofl entirely offificd at 

 the time of birth : they are more approximated, particularly 

 below, probablv from the great fize of the liver. The perfec- 

 tion in the oftification of the ribs may be explained from the 

 nature of the fun£lion in which thev are employed. Re- 

 fpiration commences at the moment of birth, and requires in 

 its organs as great a degree of perfeftion as is necefl'ary at* 

 any fubfequent age : the newly born child refpires at once 

 as it will refpire always. The organs cf locomotion, on the 

 contrary, go through a kind of education, advance very 

 flowly to perfeftion, and are, confequently, flowly deve- 

 loped. 



The fternum, which is lefs direftly concerned in refpir^ 

 tiofi, but concurs more immediately in giving folidity to the 

 chefl, is not fo much advanced as the ribs : on the contrary, 

 it is almoft entirely cartilaginous. 



The contents of the thorax undergo a great change at the 

 time of birth : the lungs, hitherto contraftcd, are diftended 

 by the admiffion of air to a much greater volume than they 

 poffeffed before ; and that part of the cheft which contains, 

 them muft be proportionally increafed. 



Towards the period of puberty, although no remarkable 

 change occurs in the ordinary feries of phenomena connefted 

 with the growth and progrefs of the bones, yet the form of 

 the cheft feems to acquire its fixed charafter. It either af- 

 fumes that lateral expanfion, and happy conformation which 

 indicate a vigorous conftitution, or the fternum projefts is 

 front, and gives the alarming prefage of a difpofition to 

 phthifis. At this time the cartilages become more denfe, 

 and the ligaments ftiffer. The motions of the ribs are more 

 confined. Hitherto they have been the chief agents in refpir- 

 ation : but in future the diaphragm is more excited. The 

 different pieces of the fternum are joined ; the ribs receive 

 more earthy matter, and become more brittle. At a later 

 period the cartilages begin to be converted into bone at 

 their centres, and oftification goes on until they are made 

 completely bo:iy. The twilling of the cartilages, which 

 we ftiall mention prefently, is now impoflible, and the upper 

 part of the cheft no longer admits of motion in its individual 

 parts. Hence in the old fubjeft relpiration is efiefted chiefly 

 by the diaphragm. 



Mechanifm of the ChcJl. — This part of the trunk has two 



principal ufes to fulfil : it protefts the included organs by 



itt folidity or power of refillance ; and it concurs by its mo- 



biUty in the various funftions of thefe organs, particularly of 



4 K 2 th* 



