' 



THE RIBS 123 



present, they are generally situated one above the other, the upper being the larger; the second 

 piece has seldom more than one; the third, fourth, and fifth pieces are often formed from two 

 centers placed laterally, the irregular union of which explains the rare occurrence of the sternal 

 foramen (Fig. 121), or of the vertical fissure which occasionally intersects this part of the bone 

 constituting the malformation known as fissura sterni; these conditions are further explained by 

 the manner in which the cartilaginous sternum is formed. More rarely still the upper end of the 

 sternum may be divided by a fissure. Union of the various centers of the body begins about 

 puberty, and proceeds from below upward (Fig. 119) ; by the age of twenty-five they are all united. 

 The xiphoid process may become joined to the body before the age of thirty, but this occurs 

 more frequently after forty; on the other hand, it sometimes remains ununited in old age. In 

 advanced life the manubrium is occasionally joined to the body by bone. When this takes place, 

 however, the bony tissue is generally only superficial, the central portion of the intervening 

 cartilage remaining unossified. 



Articulations. The sternum articulates on either side with the clavicle and upper seven costal 

 cartilages. 



The Ribs (Costse). 



The ribs are elastic arches of bone, which form a large part of the thoracic 

 skeleton. They are twelve in number oh either side; but this number may be 

 increased by the development of a cervical or lumbar rib, or may be diminished 

 to eleven. The first seven are connected behind with the vertebral column, and in 

 front, through the intervention of the costal cartilages, with the sternum (Fig. 

 115); they are called true or vertebro-sternal ribs. 1 The remaining five are false 

 ribs ; of these, the first three have their cartilages attached to the cartilage of the 

 rib above (vertebro-chondral) : the last two are free at their anterior extremities 

 and are termed floating or vertebral ribs. The ribs vary in their direction, the 

 upper ones being less oblique than the lower; the obliquity reaches its maximum 

 at the ninth rib, and gradually decreases from that rib to the twelfth. The ribs 

 are situated one below the other in such a manner that spaces called intercostal 

 spaces are left between them. The length of each space corresponds to that of 

 the adjacent ribs and their cartilages; the breadth is greater in front than behind, 

 and between the upper than the lower ribs. The ribs increase in length from 

 the first to the seventh, below which they diminish to the twelfth. In breadth 

 they decrease from above downward; in the upper ten the greatest breadth is at 

 the sternal extremity. 



Common Characteristics of the Ribs (Figs. 122, 123). A rib from the middle 

 of the series should betaken in order to study the common characteristics of these 

 bones. 



8 Each rib has two extremities, a posterior or vertebral, and an anterior or sternal, 

 and an intervening portion the body or shaft. 



Posterior Extremity. The posterior or vertebral extremity presents for examination 

 a head, neck, and tubercle. 



The head is marked by a kidney-shaped articular surface, divided by a hori- 

 zontal crest into two facets for articulation with the depression formed on the 

 bodies of two adjacent thoracic vertebras; the upper facet is the smaller; to the 

 crest is attached the interarticular ligament. } 



The neck is the flattened portion which extends lateralward from the head; it 

 is about 2.5 cm. long, and is placed in front of the transverse process of the 

 lower of the two vertebrae with which the head articulates. Its anterior surface is 

 flat and smooth, its posterior rough for the attachment of the ligament of the neck, 

 and perforated by numerous foramina. Of its two borders the superior presents a 

 rough crest (crista colli cosies') for the attachment of the anterior costotransverse 

 ligament; its inferior border is rounded. On the posterior surface at the junction 

 f the neck and body, and nearer the lower than the upper border, is an eminence 



1 Sometimes the eighth rib cartilage articulates with the sternum; this condition occurs more frequently on the 

 right than on the left side. 



