THE BONES OF THE TRUNK 47 



as between the eighth, seventh, sixth, and sometimes the fifth. 

 The eleventh and twelfth cartilages are mere nodules tipping the 

 corresponding ribs, and they have no articulation with each other, 

 nor has the eleventh with the tenth. The cartilages increase in 

 length from the first to the seventh, beyond which they gradually 

 diminish to the twelfth. They diminish in breadth from above 

 downwards. The direction of the first cartilage is inwards and 

 downwards, and that of the second horizontally inwards, whilst 

 the succeeding ones, except the eleventh and twelfth, incline more 

 and more upwards as they pass inwards. Prior to middle Hfe the 

 first costal cartilage undergoes superficial ossification underneath 

 the perichondrium, and so a thin shell of bone is formed around 

 it. In advanced hfe this condition may be met with in the other 

 costal cartilages to a certain extent. 



C. The Sternum. 



The sternum or breast-bone is situated in the middle line of 

 the anterior wall of the thorax, where it articulates on either side 

 with the first seven costal cartilages, and superiorly with the 

 clavicle. It occupies an oblique plane, which is directed downwards 

 and forwards, forming an angle with the vertical of about 20 degrees. 

 It is compressed from before backwards, of unequal width at 

 different parts, and more or less curved from above downwards, 

 the convexity being directed forwards, and being very pronounced 

 in the condition known as ' pigeon-breast.' The bone is originally 

 composed of six segments, called sternebrae. The first sternebra 

 forms the manubrium (' handle ') or presternum. The succeeding 

 four sternebrae form the body, gladiolus, or raeso-sternum, and the 

 sixth sternebra forms the ensiform or xiphoid process [xiphi- 

 sternum), otherwise known as the metasternum. 



The presternum is irregularly four-sided, and broader above 

 than below. It presents two surfaces and four borders. The 

 anterior surface is convex from side to side, and concave from above 

 downwards. It gives origin at either side to a portion of the 

 pectorahs major, and at its upper and outer part to the sternal 

 head of the sterno-cleido mastoid. Between the latter point and 

 the clavicular depression on the upper border it gives attachment to 

 the anterior sterno-clavicular ligament. The posterior surface is 

 concave. At its \ipper and outer part it gives origin to portions 

 of the sterno-hyoid and sterno-thyroid, the former being the higher 

 of the two, and close to the clavicular depression it gives attach- 

 ment to the posterior sterno-clavicular Ugament. The superior 

 border over its middle portion presents the interclavicular or semi- 

 lunar notch, which gives attachment to fibres of the interclavicular 

 ligament. At either side of this there is a large, oval, concavo- 

 convex articular surface for the clavicle, which is directed upwards, 

 outwards, and slightly backwards, an interarticular fibro-cartilage 

 intervening between the bones. Close to the inner end of each. 



