THE EIBS, THE STERNUM, AND THE THORAX 137 



The ventral- cephalic border (Fig. 96) is emarginate, and has 

 the curvature of the surfaces it separates. It is least distinct in 

 the middle region of the rib, but at the vertebral end, and especially 

 at the sternal end, it is distinct and sharp. It gives attachment to 

 the intercostal muscles. 



The dorsal-caudal border is arcuate. It begins at the tubercle, 

 in an elevated line which ends at the angle in the prominence for 

 muscular attachment. The border then becomes rounded and not 

 definitely marked as far as the sternal third, where it is sharp and 

 distinct. In its vertebral part it faces laterally, dorsally, and slightly 

 toward the tail ; in the middle, laterally ; and in the sternal part, 

 toward the tail and slightly laterally. 



The sternal extremity of the rib is squarely truncated. It 

 presents an elongated oval terminal surface, which is excavated to 

 receive the dorsal end of the costal cartilage. 



The COSTAL CARTILAGE is a curved rod about half as long as the 

 rib and much more delicate. From the dorsal to the ventral end 

 it is curved in two directions, toward the head and also medially, 

 thus making a decided rounded angle with the direction of the rib. 

 Its ends are compressed, the dorsal end in a latero-medial direction, 

 the ventral or sternal end in a cephalo-caudal direction. The dorsal 

 end fits into the end of the rib, and the ventral end presents a dorsal 

 and a ventral enlargement, which join the small tubercles on the con- 

 tiguous ends of the lateral surfaces of the fourth and fifth sternebra3. 



The ribs increase in length from the first to the ninth inclusive. 

 The tenth is equal to the ninth. The last three are progressively 

 shorter. They also increase in dorso-ventral curvature from the 

 first to the tenth, and then become straighter. 



The cephalic ribs are placed more vertically than the others, 

 which slope ventrally and toward the tail. The cephalic ribs lie 

 in vertical planes which intersect the median vertical plane of the 

 thorax almost at right angles. The planes of the middle and 

 caudal ribs intersect the vertical plane of the thorax in acute 

 angles ; in the last ribs these angles do not measure more than thirty 

 degrees. In other words, the heads of the cephalic ribs are directed 

 toward the median line of the vertebral column, while the heads of 

 the middle and caudal ribs gradually become directed more toward 



