LUNGS. 



arch projefting into the cheil, and very coiifiderably lefTen- 

 ing its extent in this direftion. But this diminution affefts 

 the middle, which is occupied by the central tendon, much 

 lefs than the fides. Again, the clavicles above very mani- 

 feftlv furmount the (lernum, and contribute to make the 

 cheii appear higher than it really is. The breadth is much 

 lefs at the upper part, than it appears to be on the firft 

 view, becaule the clavicle and the mufcles enlarge the ex- 

 terior forms without afFefting the internal dimcnfions. The 

 capacity conftantly inoreafes in proportion as we proceed 

 downwards. Yet fhe habit of wearing clothes that are very 

 tight about the waift, particularly (lays, contrafts that part, 

 fo that the cheft is fometimes (hapcd like a barrel, narrow 

 above and below, and broader in the middle. The concavity 

 in the dorfal part of the fpine makes the cheft more capa- 

 cious in its middle ; yet this enlargement is not equal to the 

 contraftion produced by the anterior prominence of the 

 bodies of the vertebras. In faft, the antero-pofterior dia- 

 meters are all much Itfs extenfive along the middle lirie of the 

 cheft than on the fides ; fo that the fternum is feparated from 

 the fpine by an interval much fmaller than that which exilts 

 between the cartilages of the ribs and the hollows at the 

 fides of the fpine. 



In the female the cheft is proportionally broader, but 

 fhorter than in the male. 



Any caufe of diftentiori afFefting the abdomen, as preg- 

 nancy, afcites, large tumours, &c. ftrongly elevates the 

 cheft, prelTes the ribs together, and diminilhes the perpen- 

 dicular axis, while the tranfverfc and antero-pofterior dia- 

 meters are rather increafed, particularly below. There are 

 malformations of the cheft, particularly affefting the (lernum 

 and ribs, which diminifh the breadth, while they leave the 

 height nearly the fame. Individuals predifpofed to phthifis 

 are remarkable for this tranfverfe contraction of the cavity, 

 which makes the prominence of the fternum very conl^piciious 

 in front. In other inllances the cheft is affected, in confe- 

 quence of deformities of the fpine : when this is curved, the 

 ribs are brought very clofe together on one fide, and are 

 proportionally feparated on the other, fo that the two fides 

 of the cavity are rendered very unequal. The chell, in fuch 

 inftauces, is generally very prominent. 



Defcrlption of the particular Bones of the Chejl. — It is com- 

 pofed of a common, and of proper parts. The dorfal por- 

 tion of the fpine is the former (fee Spixe) ; the fternum in 

 front, and the ribs on each fide, are the latter. 



The Jlernum is a fymmetrical bone, placed in the front 

 and middle of the cheft, flattened and elongated, bro?d 

 above, contradled about the middle, then a>;ain becoming 

 a little broader, and terminating at laft below by a prominent 

 point. It is divided into a cutaneous and a thoracic fur- 

 face, a clavicular and an abdominal extremity, and lateral 

 edges. 



The cutaneous furface is anterior, covered by the (kin, 

 and ir.ore immediately by the aponeurofes of the fterno- 

 tr.aftoidei and great pedloral niulcles : it is marked fome- 

 times by lines dividing it into furhices of unequal breadths, 

 and correfponding to the original divilions of the bone. It 

 is a little convex at the upper part, and then flattened. 

 The attachment of the mufcular fibres, and of the ligaments 

 of the ribs, ;;ives it a roughnefs. The thoracic furfac^ is 

 pofterior, a little concave, fmooth, and fometimes exhibits 

 tranfverfe lines fimilar to thofe already mentioned. It cor- 

 refponds above to the ilerno-hyoidei and (lerno-thyroidei ; 

 then, for a ftiort fpace, to the cellular tiffue of the mediaf- 

 tin'.-m, and afterwards to the triangularis flerni'. 



The clavicular extremity is the broadeft and thickeft part 

 of the bone. Its middle confifts of a broad concavity, al- 



moft entirely occupied by the inter-clavioular ligament : on 

 each fide of this is a large fuperficial excavation, concave 

 from within outwards, and convex from before backwards, 

 articulated to the clavicle, ar.d furrounded by ligamentous 

 infertions. The abdominal extremity is called alfo the enfi- 

 form or xiphoid cartilage or appendix. It is thin, flattened, 

 broader above, .and terminating in a narrower way below. 

 Its figure varies greatly in different individuals : the lower 

 end fometimes turns forwards, fometimes backwards ; occa- 

 fionally it is perforated. It generally is cartilaginous, in a 

 greater or lefs degree, until the later periods of life. It 

 affords infcrtion to the aponeurofes of the abdominal 

 mufcles ; the retli cover it in front, and the diaphragm be- 

 hind. 



The margins of the fternum are thick, and exhibit feven 

 articular cavities, to which the carti'ages of the true ribs are 

 articulated. Thefe a'C oblong, and not very fmooth. The 

 firft, which is fuperficial, and not clearly marked, is im- 

 mediately below the concavity that lodges the clavicle. 

 The fucceedirg ones are fiparated by flight concavities cor- 

 refponding to the intercoftal fpaces, and become nearer and 

 nearer to each other, in proportion as they are placed 

 lower. 



The fubftance of the bone is almoft entirely cellular, and 

 its furfaces are covered by a very thin compaft ftratum of 

 bony texture : hence the (ternum is very light'in proportion 

 to iis fize. It confilts at firft of ei^ht or nine piec-s, en- 

 clofed in a niafs of cartilage : thefe are foon reduced to 

 feven, and then to five ; which number continues for a long 

 time, the individual portiims bemg (lill feparated by car- 

 tilaginous ftraia. The firft of thefe pieces is the largert, 

 and is broader above than below , th-.- two following are 

 nearly fquare, and very (hort ; the fourth is longer; the laft 

 includes the enfiform appendix already mentioned. This 

 divifion no longer exifts in the adult ; the pieces are uni'ed 

 in the following order. Tlie fecond is confolidated uith the 

 third, a-id then the Litter with the fourth : the other divi- 

 fions generally cjntinue through life ; fo that the fternum is 

 ordinarily delcribed as being compofed of two bones and a 

 cartilage. The firft bone ends at the fecond rib, which is 

 artic':lated between it and the fecond bone. The two 

 pieces are united by a thin layer of cartilage, and their union 

 is often confolidated by bone. The eufiform cartilage is 

 connected in the fame w.iy to the end of the fecond bone ; 

 but after a certain age, it is generally more or lefs of- 

 fified. 



The ribs are bones of irregular figure, placed in fuccef- 

 fion from above doivnwards, on each fide of the cheft, con- 

 fiding generally, but not conftantly, of twelve pairs, flat- 

 tened and rather thin in front, rounded and thicker behind, 

 and more or lefs arched. They differ in length, breadth, 

 and direflion. The length, v.hich is incmfiderable in the 

 firlc, is fuddenly increaled to a very confidcrable degree in 

 the fecond ; and this augmentation proceeds gradually as 

 far as the eighth. From this thty again decreafe, fo that 

 the tv.'elfth is about as long as the firft. The firtt rib is the 

 broridcft ; the fucceeding ones become narrower, but in an 

 almoft infenfib'e degive. Each individual rib is narrowell 

 from its vertebral extremity to the angle: it grows broader 

 in front of this part, and incrcafes to its ftcriTai end. The 

 firft rib forms nearly a right angle with the vertebral co- 

 lumn ; the following are more and more inclined outwards 

 and downwards. Id that their vertebral are higher than tiieir 

 cartilaginous extremities. The firft forms a fmall, but 

 nearly regular femicircle ; the fucceeding ones form lefs 

 perfeft fegments of circles, which increale i'ucceffively as far 

 as the eighth, and then decreafe. All are more curved be- 

 hind 



