118 OSTEOLOGY. 



behind the angle ; a little downwards in front of it. This surface is marked by a 

 ridge which commences at the lower extremity of the head ; it is strongly marked 

 as far as the inner side of the angle, and gradually becomes lost at the junction of 

 the anterior with the middle third of the bone. The interval between it and the 

 inferior border is deeply grooved, to lodge the intercostal vessels and nerve. At 

 the back part of the bone, this groove belongs to the inferior border ; but just in 

 front of the angle, where it is deepest and broadest, it corresponds to the internal 

 surface. The superior edge of the groove is rounded ; it serves for the attachment 

 of the Internal intercostal muscle. The inferior edge corresponds to the lower 

 margin of the rib, and gives attachment to the External intercostal. Within the 

 groove are seen the orifices of numerous small foramina, which traverse the wall 

 of the shaft obliquely from before backwards. The superior border, thick and 

 rounded, is marked by an external and an internal lip, more distinct behind than 

 in front ; they serve for the attachment of the External and Internal intercostal 

 muscles. The inferior border, thin and sharp, has attached the External intercostal 

 muscle. The anterior or sternal extremity is flattened, and presents a porous, oval, 

 concave depression, into which the costal cartilage is received. 



/Structure. The ribs consist of cancellous tissue, inclosed in a thin compact layer. 



Development. Each rib, with the exception of the last two, is developed by 

 three centres, one for the shaft, one for the head, and one for the tubercle. The 

 last two have only two centres, that for the tubercle being wanting. Ossification 

 commences in the body of the ribs at a very early period, before its appearance 

 in the vertebras. The epiphysis of the head, which is of a slightly angular shape, 

 and that for the tubercle, of a lenticular form, make their appearance between the 

 sixteenth and twentieth years, and are not united to the rest of the bone until 

 about the twenty-fifth year. 



Attachment of Muscles. The Intercostals, Scalenus anticus, Scalemis medius, 

 Scalenus posticus, Pectoralis minor, Serratus magnus, Obliquus externus, Trans- 

 versalis, Quadratus lumborum, Diaphragm, Latissimus dorsi, Serratus posticus 

 superior, Serratus posticus inferior, Sacro-lumbalis, Musculus accessorius ad sacro- 

 lumbalem, Longissimus dorsi, Cervicalis ascendens, Levatores costarum. 



PECULIAR BIBS. 



The ribs which require especial consideration are five in number, viz., the first, 

 second, tenth, eleventh, and twelfth. 



The first rib (fig. 77) is one of the shortest and the most curved of all the ribs ; 

 it is broad, flat, and placed horizontally at the upper part of the thorax, its surfaces 

 looking upwards and downwards, and its borders inwards .and outwards. The 

 head is of small size, rounded, and presents only a single articular facet for arti- 

 culation with the body of the first dorsal vertebra. The neck is narrow and 

 rounded. The iuberosity, thick and prominent, rests on the outer border. There 

 is no angle, and the shaft is not twisted on its axis. The upper surface of the shaft 

 is marked by two shallow depressions, separated from one another by a ridge, 

 which becomes more prominent towards the internal border, where it terminates 

 in a tubercle ; this tubercle and ridge serve for the attachment of the Scalenus 

 anticus muscle, the groove in front of it transmitting the subclavian vein ; that 

 behind it, the subclavian artery. Between the groove for the subclavian artery 

 and the tuberosity, is a depression for the attachment of the Scalenus medius 

 muscle. The under surface is smooth, and destitute of the groove observed on the 

 other ribs. The outer border is convex, thick, and rounded. The inner, concave, 

 thin, and sharp, and marked about its centre by the tubercle before mentioned. 

 The anterior extremity is larger and thicker than in any of the other ribs. 



The second rib (fig. 78) is much longer than the first, but bears a very consider- 

 able resemblance to it in the direction of its curvature. The non-articular portion 

 of the tuberosity is occasionally only slightly marked. The angle is slight, and 

 situated close to the tuberosity, and the shaft is not twisted, so that both ends 



