196 THE SKELETON. 



its sternal extremity, an oblique line, the anterior angle. The internal surface 

 is concave, smooth, directed a little upwards behind the angle ; a little down- 



Fig. 143. Vertebral Extremity of a Rib. External Surface. 



Facet fur body of iijtpcr Dorsa 



Jiidijt f,rr fnle r-articular Liyt 

 Facet for Lof/y of louvr Dorsal Viert& 



far tra.tuv.froc. cf loitmr 



wards 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 Inter- 

 nal 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. 



Peculiar Ribs. The ribs which require especial consideration are five in 

 number, viz., the first, second, tenth, eleventh, and twelfth. 



The first rib (Fig. 144) 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 out- 

 wards. The head is of small size, rounded, and presents only a single articular 

 facet for articulation with the body of the first dorsal vertebra. The neck is 

 narrow and rounded. The tuberosity, 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 attach- 

 ment 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, an- 1 

 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 

 any of the other ribs. 



The second rib (Fig. 145) is much longer than the first, but bears a very con- 

 siderable 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 touch any plane surface upon which it may be laid. The shaft is not 

 horizontal, like that of the first rib; its outer surface, which is convex, looking 



