THORAX. 



1023 



Along the middle line, this concavity cor- 

 responds with the interval left by the diverg- 

 ence of the two pleura? (anterior mediasti- 

 num). 



In the young subject, transverse lines are 

 seen corresponding to those which occupy 

 the anterior surface ; all of these, except the 

 two between the first and second pieces of 

 the bone, are effaced at a more advanced age. 

 This surface is in relation with*many organs 

 contained in the chest, and especially the 

 heart, in front of which the sternum forms 

 a kind of shield. This is exemplified, as al- 

 ready noticed, in the frog, which is provided 

 with a sternum, though it has no ribs. At the 

 lower part of the sternum are many nutritient 

 foramina. 



Of the borders of the Sternum. These are 

 thick and marked at each side by seven an- 

 gular depressions for the reception of the 

 cartilages of the first seven ribs, which gives 

 this bone a notched and serrated appear- 

 ance. These angular cavities are separated 

 from each other by scmilunar notches, which 

 are longer above than below, where the facettes 

 closely approach each other. The uppermost 

 of these seven cavities is shallow, triangular, 

 and at an early age becomes ingrained with the 

 cartilage of the first rib ; those which follow 

 are deeper, angular, and situated at the ex- 

 tremities of each of the transverse lines. 

 When examined in the dried specimen, they 

 appear more angular and deeper in proportion 

 to the youth of the subject. 



Of the clavicular extremity. This is slightly 

 convex, and is the broadest and thickest part 

 of the whole bone. It is slightly excavated 

 from side to side, and presents at each corner 

 a depression for the reception of the sternal 

 end of the clavicle, which bears the name 

 fourchctte; this is surrounded with irregula- 

 rities for the insertion of ligaments. It fre- 

 quently happens that the two clavicular arti- 

 culations are not at the same height ; a fact 

 which was noticed by Morgagni, and which 

 Cruveilhier attributes to the unequal wearing 

 of the two articular surfaces. We have once 

 seen the clavicular articulation so low as to 

 unite with the first costal cartilage. 



Of the inferior extremity of the Sternum. 

 This is formed by the xiphoul appendix ; 

 or ensiform cartilage, for it often remains 

 cartilaginous to adult age. In length, 

 shape, and direction, it presents nume- 

 rous varieties ; it is frequently bifid, some- 

 times pierced by a foramen, and is occasion- 

 ally bent forwards, or to one side, and in 

 certain cases much depressed : its summit 

 gives attachment to an aponeurotic structure, 

 called the linca alba; behind, it indirectly 

 corresponds with the stomach, which rests 

 upon it when the body is placed in a prone 

 position. 



Connections. The sternum articulates with 

 fourteen ribs through the medium of their 

 cartilages, and more directly with the two cla- 

 vicles. 



Structure of the Sternum. This bone con- 

 sists of two very thin compact laminae, with 



an intervening cancellated structure, the cells 

 of which are very large and have very 

 delicate parietes. ,It is one of the most 

 spongy bones of the body, and it is more 

 than probable that to this circumstance the 

 frequency of disease in it may be attributed. 

 Absorption of this bone and great displace- 

 ment by bending inwards is very common, 

 particularly in women who wear tight stays. 

 Under such circumstances, or by disease, we 

 have witnessed the sternum so depressed in- 

 wards that the depth, including all the thoracic 

 integuments from the spinous processes of 

 the dorsal vertebrae to the anterior surface 

 of this bone, did not exceed three inches, in- 

 stead of from seven to nine inches. 



Of the Development or Ossification of the 

 Sternum. As far as the middle of foetal 

 life or a little later, the sternum is altogether 

 cartilaginous, as represented ata,Jig. 660. This 



Fig. 660. 



bone is one of the slowest in its ossification ; 

 it exhibits no bony points or centres of os- 

 sification up to the sixth month of foetal 

 life. It is also of all bones the one in which 

 the phenomena of ossification proceed with 

 the least regularity. After the sixth month 

 of foetal life, ossification begins with the for- 

 mation of osseous granules in the middle of 

 the intervals between the points at which the 

 cartilages of the ribs are connected. 



" There are five of these granules for the 

 sternum, exclusive of the ensiform appendage, 

 and they form as many pieces (e, Jig. 660.). 

 The process of ossification makes its appear- 

 ance in the first between the fifth and sixth 

 months ; and, soon following in the second and 

 third, it reaches the fourth at the end of foetal 

 life. The osseous centre of the last varies 

 considerably in the time of its appearance. 

 It may be found soon after birth, and may 

 not be visible for a considerable time (one or 

 two years) after that period." 



" In many cases, one or more of the divi- 

 sions of the sternum are formed from more 

 nuclei than one, and there are peculiarities 

 with respect to the number and position of 

 these additional granules which require notice.'* 



Ossification of the 1st piece. This some- 

 times presents a single nucleus (Jig. 660, b 1.), 

 rounded and transversely strong ; sometimes 

 it presents two nuclei, and in this case they 



