THE RIBS 163 



affords attachment to the posterior costotransverse ligament. The tubercle is 

 much more prominent in the upper than in the lower ribs. 



Anterior Extremity. The anterior or sternal extremity is flattened, and presents 

 a porous, oval, concave depression, into which the costal cartilage is received. 



The shaft (corpus costae) is thin and flat, so as to present two surfaces, an 

 external and an internal, and two borders, a superior and an inferior. 



The external surface is convex, smooth, and marked at its back part, a little 

 in front of the tuberosity, by a prominent line, directed obliquely from above 

 downward and outward; this gives attachment to a tendon of the Iliocostalis 

 muscle or of one of its accessory portions, and is called the angle (anguhis costae). 

 At this point the rib is bent in two directions. If the rib is laid upon its lower 

 border, it will be seen that the portion of the shaft in front of the angle rests upon 

 this border, while the portion of the shaft behind the angle is bent inward and at 

 the same time tilted upward. The interval between the angle and the tuberosity 

 increases gradually from the second to the tenth rib. The portion of bone between 

 these two parts is rounded, rough, and irregular, and serves for the attachment of 

 the Longissimus dorsi muscle. The portion of bone between the tubercle and 

 sternal extremity is also slightly twisted upon its own axis, the external surface 

 looking downward behind the angle, a little upward in front of it. This surface 

 presents toward its sternal extremity an oblique line, the anterior angle. 



The internal surface is concave, smooth, directed a little upward behind the angle, 

 a little downward in front of it. This surface is marked by a ridge which com- 

 mences 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 

 presents a groove, subcostal groove (sulcus costae), for 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 

 muscle. Within the groove are seen the orifices of numerous small foramina 

 which traverse the wall of the shaft obliquely from before backward. 



The superior border, thick and rounded, is marked by an external and an inter- 

 nal 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 to it the External intercostal 

 muscle. 



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

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



First Rib. The first rib (Fig. 131) is the shortest and the most curved of all 

 the ribs; it is broad and flat, its surfaces looking upward and downward, and 

 its borders inward and outward. The head is of small size, rounded, and presents 

 only a single articular facet for articulation with the body of the first thoracic ver- 

 tebra. The neck is narrow and rounded. The tuberosity, thick and prominent, 

 rests on the outer border. There is no angle, but in this situation the rib is slightly 

 bent, with the convexity of the bend upward, so that the head of the bone is 

 directed downward. The upper surface of the shaft is marked by two shallow 

 depressions, separated by a small rough surface (tuberculum scaleni) for the attach- 

 ment of the Scalenus anticus muscle the shallow groove in front of it trans- 

 mitting the subclavian vein, the deeper groove behind it (sulcus subclaviae) 

 the subclavian artery. Between the groove for the subclavian artery and the 

 tuberosity is a rough surface, for the attachment of the Scalenus medius muscle. 

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



