THE BONES OF THE TRUNK 43 



two borders. The external surface is convex, and its plane is vertical 

 behind, but oblique in front, being here sloped downwards and 

 fonsards. Opposite the greatest bend of the rib it presents an 

 oblique ridge, directed do%vnwards and outwards, called the angle, 

 for a tendinous shp of the erector spinje. The surface between 

 the tubercle and the angle gives attachment to the longissimus 

 dorsi. Near to the anterior extremity (about 2 inches from it) the 

 external surface presents another obhque ridge directed downwards 

 and outwards, knoNXTi as the anterior angle, where the rib describes 

 a slight curve. The internal surface, which is concave, is covered 

 by the costal pleura. At its lower part it presents the subcostal 

 groove, to be presently described. The superior border is thick 

 and round behind, but thin and sharp in front. Its outer hp gives 

 attachment to an external intercostal muscle, and its inner Up to 

 an internal intercostal, a collateral intercostal artery Ijdng between 

 the two muscles. The inferior border is for the most part sharp and 

 wiry. Immediately within and above it there is the subcostal 

 groove, which conmiences behind at the tubercle and disappears 

 over the anterior fifth. Posteriorly the groove belongs to the 

 inferior border. Its upper hp is rovmded and gives attachment to 

 an internal intercostal muscle, whilst the lower hp gives attach- 

 ment to an external intercostal. The nutrient foramen is situated 

 in the subcostal groove a httle anterior to the centre of the bone, 

 and the canal to which it leads is directed towards the head. It 

 gives passage to a branch of the intercostal artery which lies in 

 the groove. The contents of the groove from above downwards 

 are an intercostal vein, artery, and nerve. 



The interior or sternal extremity presents an oval pit, which is 

 almost vertical in direction, for the costal cartilage. 



The Peculiar Ribs. — ^These are the first, second, tenth, eleventh 

 and twelfth. 



The First Rib. — ^This is the shortest, broadest, and flattest of 

 all the ribs, and its curs'e is very distinct, but there is no twist. 

 The head is small, and presents a nearly circular facet for articula- 

 tion with the entire facet on the body of the first thoracic vertebra. 

 The neck is narrow, and compressed from above downwards. The 

 tubercle, which is large, is situated on the external border at the 

 junction of the neck with the shaft. Being placed opposite the 

 greatest bend of the bone, it takes the place of the angle, and pre- 

 sents the usual articular and non-articular portions, the former 

 being for the first thoracic transverse process. 



The shaft is broad and compressed from above downwards, its 

 surfaces being superior and inferior, whilst the borders are internal 

 and external. The superior surface close to the anterior extremity 

 gives attachment to the tendon of the subclavius and the 

 costo-coracoid ligament. Farther back there are two oblique 

 grooves, separated to a limited extent by a tubercle or spine 

 for the scalenus anticus. The anterior groove is shallow, and 

 lodges the subclavian vein, whilst the posterior deeper groove 



