Thorax 



59 



dividing ridge extending between the two sixth cartilages. The pseudo-ridge cannot be of the same 

 nature as the other transverse ridges, for these mark the line of junction of the sternebrz, of which 

 each one is developed from distinct centres. The paired centres for the last segment but one 

 appear shortly before birth, and those for the last segment within a year after birth, and no further 

 centres occur in the body, so that there cannot be an additional true transverse ridge. The meta- 

 sternum commences to ossify some years later than the last segment. 



The posterior surface of the body is covered by a very thick layer of vertically 

 arranged aponeurotic fibres, which extend down on to the ensiform process. The 

 layer is not shown in the figure, but it separates the bone from the pleural sacs, whose 

 anterior edges are shown by the interrupted lines. At the margins of the body and 

 xiphoid, from the level of the fourth cartilages downward, the Triangularis sterni is 

 inserted. This muscle arises from the cartilages as far up as the second ; its upper 

 fibres, as indicated in the drawing, are very oblique, becoming less so as they get lower, 

 until it merges into the Transversalis abdominis (Fig. 46). 



The Diaphragm arises by two muscular slips from the back of the xiphisternum. 



On the manubrium there is no thick membrane. The Sterno-thyroids arise by a 

 curved line of continuous origin along the lower part, from one first cartilage to the 

 other, and they lie directly against the bone. Observe that the right muscle as a rule 

 transgresses the middle line in its area of origin. 



The Sterno-hyoids arise by a much smaller origin near the top of the bone. None 

 of these infra-hyoid muscles arise by tendon, so no ridges on the bone mark their 

 situation. The deep layer of the " omohyoid fascia " reaches the bone below the 

 Sterno-thyroids, with the pretracheal fascia, just at or above the angle : the superficial 

 layer of the omohyoid fascia is attached to the upper and back part, above the 

 Sterno-hyoids. 



The internal mammary artery is shown in the figure, running down immediately 

 outside the Sterno-thyroids, and giving branches between the muscles and the bone : 

 these branches give vessels into the numerous foramina on the back of the manubrium. 

 Lower down the artery gives branches to the back of the body, and even when it lies 

 in front of Triangularis sterni some smaller twigs may come through that structure. 

 Some of these vessels may pass at once deep to the outer edge of the posterior membrane, 

 but the larger branches appear to pass through foramina of some size in the membrane 

 to get to the bone. 



The disposition of the membrane suggests that it is the degenerated remnant of a muscular 

 sheet, of which the upper part only remains now in the infra-hyoid muscles attached to the 

 manubrium. 



The sternum consists of a thin shell of compact bone enclosing a coarse cancellous 

 tissue, the meshes of which are filled with red marrow : the other bones of the thorax, 

 ribs and vertebrae, also contain red marrow. 



The male sternum is longer in proportion to its breadth than the female bone : 

 the difference in length is mainly in the body of the bone, the female body being less 

 than twice the length of the manubrium, while in the male it is more than twice the 

 length, but part of this difference depends on the fact that the male manubrium is on 

 the whole somewhat smaller proportionately than in women. 



The bone slopes slightly forwards as well as downwards in the body, forming with 

 the vertebral column an angle of 20 to 25 degrees, so that the antero-posterior depth 

 of the chest is considerably greater at the lower end of the sternum than at the level of 

 the manubrium, in accordance with the position and shape of the heart and peri- 

 cardium. Although it is covered by the great pectoral muscle in the greater part 

 of its extent, yet, as this muscle is thin on the bone, the sternum is practically palpable 



