THE CHEST. 245 



to tde sevcntli rib ; and beliiiul and below tliat there is tlie bony structure 

 of tbe ribs, of no little strength ; and their arched form, although a 

 flattened arch ; and the yielding motion at the base of each rib, resulting 

 from its jointed connection with the spine above and its cartilaginous union 

 Avith the sternum below. 



A still more important consideration with regard to the parietes of the 

 thorax is the manner in wliich they can adapt themselves to the changing 

 bulk of the contents of the cavity. The capacity of the chest is little 

 affected by the external contraction and dilatation of the heart, for when 

 its ventricles are collapsed its auricles are distended, and when its auricles 

 are compressed its ventricles expand ; but with regard to the lungs it is a 

 very different affair. In their state of collapse and expansion they vary in 

 comparative bulk, one-sixth part or more, and, in either state, it is necessary 

 for the proper discharge of the function of respiration that the parietes of 

 the chest should be in contact with them. 



The ribs are eighteen in number on either side. Eight of them are 

 perfect, and commonly called the true, or, more properly, sternal ribs, 

 extending from the spine to the sternum. The remaining ten are posterior 

 and shorter, and are only indirectly connected with the sternum. 



The ribs are united to the corresponding vertebrae or bones of the spine, 

 so as to form perfect joints — or rather, each rib forms two joints. The 

 head of the rib is received between the vertebrae or bones of the spine, 

 liefore and behind, so that it shall always present two articulating surfaces, 

 one opposed to the verterbra immediately before, and the other to that 

 immediately behind, and each forming a distinct joint, with a perfect 

 capsular ligament, and admitting of a rotatory motion. The tubercle of 

 the rib seems to be received into the cartilaginous ligamentous substance 

 betAveen the vertebrae, and is articulated to the transverse process of the 

 posterior vertebrae connected with the head. Nothing oould be more 

 admirably devised for motion, so far as it is required, and for strength of 

 union, that can scarcely be broken. 



Before the ribs reach the sternum, they terminate in a cartilaginous 

 prolongation, or the lower part of the rib may be said to be cartilaginous ; 

 and where it unites wdth the sternum there is a third joint, with a perfect 

 and complete capsular ligament. 



The cartilage of the posterior ribs are also united to the bony portion. 

 They are not, however, prolonged so far as the sternum; but the extremity 

 of one lies upon the body of that which is immediately before it, bound down 

 upon it by a cellular substance approaching to the nature of ligament, yet 

 each having some separate motion, and all of them connected indirectly with 

 the sternum by means of the last sternal rib. It is an admii'able contrivance 

 to preserve the requisite motion which must attend every act of breathing, 

 every extension and contraction of the chest, Avith a degree of strength 

 Avhich scarcely any accident can bi-eak through. 



TJie Sternum,, or bi'east-bone, is more complicated than it at first appears 

 to be. It constitutes the floor of the chest, and is a long flat spongy bone, 

 fixed betAveen the ribs on either side, articulating Avith their cartilages, and 

 serving as a point of support to them. It is composed of from seven to 

 nine pieces, united together by cai-tilage ; and AvhatcA^er changes may take 

 place in other parts of the frame, tins cartilage is not converted to bone 

 even in extreme old age, although there may, possibly, be some spots of 

 ossific matter found in it. 



The point of the breast-bone ma}'" be occasionally injured by blows or by 

 the pressure of the collar. It has been, by brutal Aaolence, completely 

 broken off from the sternum ; but oftener, and that from some cruel usage, 



