100 STERNUM. 



THORAX AND UPPER EXTREMITY. 



The bones of the thorax are the sternum and ribs ; and, of the upper 

 extremity, the clavicle, scapula, humerus, ulna and radius, bones of the 

 carpus, metacarpus, and phalanges. 



STERNUM. The sternum (fig. 49) is situated L the middle line of the 

 front of the chest, and is oblique in direction, the superior end lying within 

 a few inches of the vertebral column, and the inferior being projected for- 

 wards so as to be placed at a considerable distance from the spine. The 

 bone is flat or slightly concave in front, and marked by five transverse 

 lines which indicate its original subdivision into six pieces. It is convex 

 behind, broad and thick above, flattened and pointed below, and is divisible 

 in the adult into three pieces, superior, middle, and inferior. 



The superior piece , or rnanubrium, is nearly quadrilateral ; it is broad 

 and thick above, where it presents a concave border (incisura semilu- 

 naris), and narrow at its junction with the middle piece. At each supe- 

 rior angle is a deep articular depression (incisura clavicularis) for the 

 clavicle, and on either side two notches, for the articulation of the carti- 

 lage of the first rib, and one half of the second. 



The middle piece, or body, considerably longer than the superior, is 

 broad in the middle, and somewhat narrower at each extremity. It pre- 

 sents at either side six articular notches, for the lower half of the second 

 rib, the four next ribs, and the upper half of the seventh. This piece is 

 sometimes perforated by an opening of various magnitude, resulting from 

 arrest of development. 



The inferior piece (ensiform or xiphoid cartilage) is the smallest of the 

 three, often merely cartilaginous, and very various in appearance, being 

 sometimes pointed, at other times broad and thin, and at other times 

 again, perforated by a round hole, or bifid. It presents a notch at 

 each side for the articulation of the lower half of the cartilage of the 

 seventh rib. 



Development. By a variable number of centres, generally ten, namely, 

 two for the rnanubrium ; one (sometimes two) for the first piece of the 

 body, two for each of the remaining pieces, and one for the ensiform car- 

 tilage. Ossification commences towards the end of the fifth month in the 

 manubrium, the two pieces for this part being placed one above the other. 

 At about the same time the centres for the first and second pieces of the 

 body are apparent ; the centres for the third piece of the body appear a 

 few months later, and those for the fourth piece soon after birth. The 

 osseous centre for the ensiform cartilage is so variable in its advent, that 

 it may be present at any period between the third and eighteenth year. 

 The double centres for the body of the sternum are disposed side by side 

 in pairs, and it is the irregular union of these pairs in the last three pieces 

 of the body that gives rise to the large aperture occasionally seen in the 

 sternum, towards its lower part. Union of the pieces of the sternum 

 commences from below and proceeds upwards ; the fourth and the third 

 unite at about puberty, the third and the second between twenty and 

 twenty-five, and the second and the first between twenty-five and thirty. 

 The ensiform appendix becomes joined to the body of the sternum at 

 forty or fifty years ; and the manubrium to the body only in very old age. 

 Two small pisiform pieces have been described by Beclard and Breschet, 



