THE RIBS, THE STERNUM, AND THE THORAX 133 



join the sternum. The ventral ends of the cephalic three pairs of 

 these the tenth, the eleventh, and the twelfth are indirectly 

 attached to the sternum through the union of their ventral parts 

 with one another and with the ventral part of the ninth pair. The 

 ventral ends of the thirteenth pair are more free, and are embedded 

 in the muscles of the abdominal wall ; hence these ribs are termed 

 floating ribs. 



The true ribs are also called vertebro-sternal ribs ; the false 

 ribs, asternal ribs ; and the floating ribs, vertebral ribs. 



Each rib is formed of two distinct elements, a longer, dorsal, bony 

 portion, to which the name rib is commonly restricted, and a shorter, 

 ventral, cartilaginous portion, known as the costal cartilage. The 

 bony ribs form the dorsum and most of the sides of the thorax ; 

 the costal cartilages with the sternum constitute the ventral wall and 

 the ventral part of the sides. 



The ribs increase in length from the cephalic end of the thorax, 

 and the costal cartilages rise more dorsally on the side-walls of the 

 thorax at the caudal end than they do at the cephalic end. 



CHARACTERS OF A TYPICAL RIB. 



The Fifth Rib (Figs. 95, 96) may be selected for study, since it lies 

 in the middle of the thoracic wall and thus presents an average size. 

 It is also in relation dorsally with an average thoracic vertebra and 

 ventrally with an average sternebra. We shall consider first the rib 

 and then the costal cartilage. 



The RIB (Fig. 95) is a slender, curved bar, about ten times as long- 

 as wide. Its ventral or sternal half is flattened and slightly curved. 

 Its dorsal or vertebral half is thicker and strongly curved. The rib 

 presents a head separated by a neck from the shaft. 



The Head is the rounded, vertebral extremity of the bone. It is 

 very little wider or higher than the neck, and presents a smooth, semi- 

 spherical surface, which faces medially, dorsally, and toward the head 

 and articulates with the bodies of the fourth and fifth vertebrae. A 

 vertical ridge with a central pit divides the articular surface of the 

 head into two convex facets, whereof the cephalic and larger articu- 

 lates with the half-facet on the bodv of the fourth vertebra and the 



t/ 



caudal and smaller articulates with the half-facet on the body of the 

 fifth vertebra. A strong ligament from the intervertebral disk is 



