THE RIBS 127 



termed true or sternal ribs. The remaining five pairs, known as false or asternal 

 ribs, may be arranged in two sets: — one, including the eighth, ninth, and tenth 

 ribs, in which the cartilages of the anterior extremities are connected together, 

 and the other, including the eleventh and twelfth, in which the anterior extremi- 

 ties, tipped with cartilage, are free. The eleventh and twelfth are known, in 

 consequence, as the floating ribs. Thus, the first seven are vertebro -sternal; 

 the eighth, ninth, and tenth, vertebro -chondral; the eleventh and twelfth, 

 vertebral ribs. 



The ribs increase in length from the first to the seventh, and decrease from the 

 seventh to the twelfth. They also vary in their direction, the upper ones being 

 less oblique than the lower. The obliquity is greatest at the ninth rib and gradu- 

 ally decreases from the ninth to the twelfth. 



Typical characters of a rib (fig. 154). — The seventh is regarded as "the most 

 typical rib. It presents for examination a vertebral extremity or head; a narrow 

 portion or neck; a sternal extremity; and an intermediate portion, the body or 

 shaft. 



The head [capitulum costse] presents an articular surface made up of two 

 articular facets separated by a horizontal crest [crista capituli]. The crest is 

 connected by an interarticular ligament with an intervertebral disc, and the facets 

 articulate mth the costal pits on the sides of the bodies of two vertebrae (sixth and 

 seventh) . As a rule, the lower facet is the larger, and articulates with the thoracic 

 vertebra, to which the rib corresponds in number. This is the primary facet, and 

 is the one represented in those ribs which possess only a single facet on the rib-head. 

 The anterior margin is lipped for the attachment of the radiate ligament. 



The neck [coUum costse] is that portion of the rib extending from the head to 

 the tubercle. It is flattened from before backward and the posterior surface is 

 in relation with the transverse process of the lower of the two vertebrae wdth which 

 the head articulates; it forms the anterior boundary of the costo-transverse fora- 

 men, and is rough where it is attached to the neck (middle costo-transverse) 

 ligament. The anterior surface is flat and smooth. The superior border of the 

 neck, continuous with the corresponding border of the shaft, presents a rough 

 crest [crista colli] for the anterior costo-transverse ligament. The inferior bor- 

 der of the neck is rounded and continuous with the ridge of the costal groove. 

 This difiference in the relation of the neck, to the upper and lower borders of the 

 rib-shaft, is useful in determining to which side a rib belongs. 



The tubercle, situated behind at the junction of the neck with the shaft, con- 

 sists of an upper and lateral part, rough for the attachment of the posterior costo- 

 transverse ligament, and a lower and medial part, bearing a facet for articulation 

 with a pit near the tip of the transverse process. The tubercle projects 

 below the lower edge of the rib to form a crest, marking the beginning of the costal 

 groove. 



The body is strongly curved and presents for examination two surfaces and 

 two borders. At first the curve is in the same plane as the neck, but it quickly 

 turns forward at a spot on the posterior surface of the shaft known as the angle, 

 where it gives attachment to the ilio-costalis muscle and some of its subdivisions. 

 The rib has also a second or upward curve, beginning at the angle. These curves 

 are expressed by describing the main curve as disposed around a vertical, and the 

 second or upward curve around a second transverse axis. 



When a rib, except the first, second, and twelfth, is laid with its lower border upon the table, 

 the rib-head rises and the rib touches the table at two places, viz., at the anterior end, and in the 

 neighbourhood of the angle. 



Besides the two curves now described, the rib is slightly twisted on itself, so that the sur- 

 faces which look medially and laterally behind are placed obliquely in front and look downward 

 as well as medially, and upward as well as laterally. 



The external surface of the rib is convex, and gives attachment to muscles. Near its an- 

 terior extremity it forms a somewhat abrupt curve, indicated by a ridge on the bone, which gives 

 attachment to the serratus anterior (niagnus) muscle, and is sometimes caUed the anterior 

 angle. 



The internal surface is concave and presents near its inferior border the 

 costal groove [sulcus costse]. The groove is best marked near the angle, and 

 gradually becomes shallower toward the anterior extremity of the rib, where it 

 is finally lost; it lodges the intercostal vessels and nerve. The ridge Umiting the 



