THE RIBS, THE STERNUM, AND THE THORAX 135 



not prominent, but is a little larger than the surface from which it 

 rises. Its entire dorsal surface is occupied by a smooth facet, which 

 articulates with the facet on the ventral surface of the transverse 

 process of the fifth vertebra. The articular surface is gently saddle- 

 shaped, concave from end to end, and convex from the cephalic to the 

 caudal edge. It faces dorsally and toward the tail. 



The Shaft of the rib comprises the rest of the bone. Its vertebral 

 part curves strongly laterally and ventrally to a point dorsal to the 

 junction of the dorsal with the middle third, where it becomes less 

 arched and is directed more vertically. The point at which this 

 change in the dorso-ventral curvature takes place is known as the 

 angle of the rib, and is usually marked by a roughened prominence, 

 sometimes elevated into a decided tubercle, for the attachment of the 

 ilio-costal muscle. 



At the angle, the rib makes a slight bend ventrally and toward the 

 tail. The angle also marks the point at which the rib begins to be 

 twisted about its own long axis. Dorsal to the angle it is flattened 

 from edge to edge and presents cephalic and caudal surfaces separated 

 by dorsal and ventral borders. Below the angle the rib becomes 

 rounded and then gradually flattened from without inward, and the 

 cephalic surface gradually turns, to become entirely lateral at the 

 ventral end. The caudal surface becomes medial, and the dorsal 

 and ventral borders become the caudal and cephalic borders re- 

 spectively. 



The caudal-medial surface (Fig. 95) is the continuation of the 

 caudal and ventral surfaces of the neck. It is concave dorso-ventrally 

 for most of its extent, but is almost flat in its sternal part. It is more 

 convex in the vertebral part than in the sternal part, and narrower in 

 the middle. It is divided into definite vertebral and sternal parts by 

 a ridge, or line of maximum convexity, beginning as the continuation 

 of the caudal-ventral border of the neck, arching ventrally and later- 

 ally, and joining the dorsal-caudal border in its sternal third. The 

 vertebral part is the continuation of the caudal surface of the neck. 

 It is a curved triangular strip, wide at the vertebral end and pointed 

 at the sternal end. It is depressed or even slightly excavated at its 

 vertebral end, but, in the main, flat or somewhat convex from dorsal 

 to ventral border. It faces toward the tail, and, in a certain degree, 

 medially. On it lie the intercostal vessels. The sternal part of the 



